fix(i18n): wrap hard-coded English strings in t() across UI (#6566)

## Summary
Audit + bulk fix of hard-coded English UI strings - `aria-label`,
`title`, `placeholder`, `label`, and raw JSX literals that bypassed i18n
entirely. Each literal now goes through `t("key", "English Default")`
from `react-i18next`, and every new key has a corresponding entry in
`en-GB/translation.toml` so translators can pick it up.

## What this fixes
Strings were rendered untranslated in every non-EN locale because they
never went through `t()` at all (not just "value not translated yet").
Affects screen-reader labels, tooltips, form placeholders, empty/loading
states, plan card content, and the entire workflow ParticipantView.

## Coverage (~143 keys / 50 files)
- **Viewer chrome** - search bar (close, clear, prev/next, "of N"
results), link/signature/redaction actions, viewer error state, zoom
labels
- **Page editor** - undo/redo/rotate/delete toolbar tooltips, empty
state, bulk selection operator chip tooltips
- **Shared primitives** - Tooltip close, InfoBanner dismiss, TextInput
clear, Toast dismiss/toggle, UpdateModal close, EditableSecretField
edit, DropdownListWithFooter search, FileCard/FileDropdownMenu actions,
EmptyFilesState + AddFileCard upload
- **Tools** - Image upload + hint, ColorControl eyedropper, sign Use
Signature, CompressSettings, OCR loading, PageLayout
margin/border/row/col placeholders, FormFill switch + save + re-scan
- **Proprietary admin** - OverviewHeader signed-in line + logout,
AdminPremiumSection moved-features list (via `<Trans>`),
AdminPlanSection no-data alert, AdminAdvancedSection temp-dir
placeholders, AdminEndpointsSection multiselect placeholders,
AdminMailSection + AdminDatabaseSection password placeholders
- **Onboarding** - MFASetupSlide QR loading + auth code label,
SecurityCheckSlide role select + options
- **ParticipantView** - entire sign-document UI (~30 strings: loading,
error, badges, headings, cert-type Select, all input labels and
placeholders, action buttons, completion + expired alerts) - file
previously imported `useTranslation` but only used `t()` for cert
validation
- **planConstants.ts refactor** - replaced `PLAN_FEATURES` /
`PLAN_HIGHLIGHTS` const exports with `usePlanFeatures()` /
`usePlanHighlights()` hooks. Service layer (`licenseService.getPlans`)
updated to accept feature/highlight maps so it stays hook-free. Callers
(`usePlans`, `CheckoutContext`) resolve the hooks at the React boundary
- **Previously catalogued offenders** - `FileSidebarFileItem`
open/close-viewer aria-labels, `quickAccessBar/ActiveToolButton` "Back
to all tools" tooltip + aria, `AppConfigModal` close button

## Notes
- One small refactor in `usePageSelectionTips.ts` was needed to resolve
a TOML key-shape conflict: the existing scalar keys
`bulkSelection.operators.{and,not,comma}` needed to become tables to
hold the new `.title` subkeys for OperatorsSection's chip tooltips. The
existing descriptions moved to `[bulkSelection.operators.descriptions]`
and the three i18n key paths in usePageSelectionTips were updated to
match.
- Viewer sidebar close buttons
(Bookmark/Layer/Thumbnail/Attachment/Comments) were on the audit list
but are NOT on main - they're added by the unmerged PR #6552
(feat/viewer-sidebar-ux). Those particular strings will need wrapping
when that PR lands.
- TOML hook (`toml-sort-fix`) ran and re-sorted the translation file.

## Test plan
- [ ] `task frontend:typecheck` passes (core + proprietary + desktop
variants)
- [ ] `task frontend:lint` passes
- [ ] Switching language to Deutsch / Русский: previously-English
`aria-label`s + tooltips + placeholders + plan card bullets now render
translated (when the locale has values) or fall back to the English
default (when it doesn't)
- [ ] Plan page bullet points in EN render unchanged
- [ ] Sign-document flow (ParticipantView) renders unchanged in EN

frontend/editor/src/proprietary/components/shared/config/OverviewHeader.tsx

@@ -39,13 +39,15 @@ export function OverviewHeader() {
           </Text>
           {user?.email && (
             <Text size="xs" c="dimmed" mt="0.25rem">
-              Signed in as: {user.email}
+              {t("account.overview.signedInAs", "Signed in as: {{email}}", {
+                email: user.email,
+              })}
             </Text>
           )}
         </div>
         {user && (
           <Button color="red" variant="filled" onClick={handleLogout}>
-            Log out
+            {t("account.overview.logOut", "Log out")}
           </Button>
         )}
       </div>

frontend/editor/src/proprietary/components/shared/config/configSections/AdminAdvancedSection.tsx

@@ -809,7 +809,10 @@ export default function AdminAdvancedSection() {
                     },
                   })
                 }
-                placeholder="Default: java.io.tmpdir/stirling-pdf"
+                placeholder={t(
+                  "admin.settings.advanced.tempFileManagement.baseTmpDir.placeholder",
+                  "Default: java.io.tmpdir/stirling-pdf",
+                )}
                 disabled={!loginEnabled}
               />
             </div>
@@ -834,7 +837,10 @@ export default function AdminAdvancedSection() {
                     },
                   })
                 }
-                placeholder="Default: baseTmpDir/libreoffice"
+                placeholder={t(
+                  "admin.settings.advanced.tempFileManagement.libreofficeDir.placeholder",
+                  "Default: baseTmpDir/libreoffice",
+                )}
                 disabled={!loginEnabled}
               />
             </div>
@@ -859,7 +865,10 @@ export default function AdminAdvancedSection() {
                     },
                   })
                 }
-                placeholder="System temp directory path"
+                placeholder={t(
+                  "admin.settings.advanced.tempFileManagement.systemTempDir.placeholder",
+                  "System temp directory path",
+                )}
                 disabled={!loginEnabled}
               />
             </div>

frontend/editor/src/proprietary/components/shared/config/configSections/AdminDatabaseSection.tsx

@@ -698,7 +698,10 @@ export default function AdminDatabaseSection() {
                     onChange={(value) =>
                       setSettings({ ...settings, password: value })
                     }
-                    placeholder="Enter database password"
+                    placeholder={t(
+                      "admin.settings.database.password.placeholder",
+                      "Enter database password",
+                    )}
                     disabled={!loginEnabled}
                   />
                 </div>

frontend/editor/src/proprietary/components/shared/config/configSections/AdminEndpointsSection.tsx

@@ -296,7 +296,10 @@ export default function AdminEndpointsSection() {
                 }))}
                 searchable
                 clearable
-                placeholder="Select endpoints to disable"
+                placeholder={t(
+                  "admin.settings.endpoints.toRemove.placeholder",
+                  "Select endpoints to disable",
+                )}
                 comboboxProps={{ zIndex: 1400 }}
                 disabled={!loginEnabled}
               />
@@ -330,7 +333,10 @@ export default function AdminEndpointsSection() {
                 }))}
                 searchable
                 clearable
-                placeholder="Select groups to disable"
+                placeholder={t(
+                  "admin.settings.endpoints.groupsToRemove.placeholder",
+                  "Select groups to disable",
+                )}
                 comboboxProps={{ zIndex: 1400 }}
                 disabled={!loginEnabled}
               />

frontend/editor/src/proprietary/components/shared/config/configSections/AdminMailSection.tsx

@@ -286,7 +286,10 @@ export default function AdminMailSection() {
                 onChange={(value) =>
                   setSettings({ ...settings, password: value })
                 }
-                placeholder="Enter SMTP password"
+                placeholder={t(
+                  "admin.settings.mail.password.placeholder",
+                  "Enter SMTP password",
+                )}
               />
             </div>
 

frontend/editor/src/proprietary/components/shared/config/configSections/AdminPlanSection.tsx

@@ -183,8 +183,14 @@ const AdminPlanSection: React.FC = () => {
 
   if (!plans || plans.length === 0) {
     return (
-      <Alert color="yellow" title="No data available">
-        Plans data is not available at the moment.
+      <Alert
+        color="yellow"
+        title={t("admin.settings.plan.noData.title", "No data available")}
+      >
+        {t(
+          "admin.settings.plan.noData.message",
+          "Plans data is not available at the moment.",
+        )}
       </Alert>
     );
   }

frontend/editor/src/proprietary/components/shared/config/configSections/AdminPremiumSection.tsx

@@ -1,5 +1,5 @@
 import { useCallback, useEffect } from "react";
-import { useTranslation } from "react-i18next";
+import { Trans, useTranslation } from "react-i18next";
 import {
   TextInput,
   Switch,
@@ -132,17 +132,29 @@ export default function AdminPremiumSection() {
             </List.Item>
             <List.Item>
               <Text size="sm" component="span">
-                <strong>Custom Metadata</strong> (PRO) - General
+                <Trans
+                  i18nKey="admin.settings.premium.movedFeatures.customMetadata"
+                  defaults="<0>Custom Metadata</0> (PRO) - General"
+                  components={[<strong />]}
+                />
               </Text>
             </List.Item>
             <List.Item>
               <Text size="sm" component="span">
-                <strong>Audit Logging</strong> (ENTERPRISE) - Security
+                <Trans
+                  i18nKey="admin.settings.premium.movedFeatures.auditLogging"
+                  defaults="<0>Audit Logging</0> (ENTERPRISE) - Security"
+                  components={[<strong />]}
+                />
               </Text>
             </List.Item>
             <List.Item>
               <Text size="sm" component="span">
-                <strong>Database Configuration</strong> (ENTERPRISE) - Database
+                <Trans
+                  i18nKey="admin.settings.premium.movedFeatures.databaseConfiguration"
+                  defaults="<0>Database Configuration</0> (ENTERPRISE) - Database"
+                  components={[<strong />]}
+                />
               </Text>
             </List.Item>
           </List>

frontend/editor/src/proprietary/components/shared/config/configSections/plan/StaticPlanSection.tsx

@@ -11,7 +11,10 @@ import {
 import { useTranslation } from "react-i18next";
 import { alert } from "@app/components/toast";
 import { LicenseInfo, mapLicenseToTier } from "@app/services/licenseService";
-import { PLAN_FEATURES, PLAN_HIGHLIGHTS } from "@app/constants/planConstants";
+import {
+  usePlanFeatures,
+  usePlanHighlights,
+} from "@app/constants/planConstants";
 import FeatureComparisonTable from "@app/components/shared/config/configSections/plan/FeatureComparisonTable";
 import StaticCheckoutModal from "@app/components/shared/config/configSections/plan/StaticCheckoutModal";
 import LicenseKeySection from "@app/components/shared/config/configSections/plan/LicenseKeySection";
@@ -32,6 +35,8 @@ const StaticPlanSection: React.FC<StaticPlanSectionProps> = ({
   currentLicenseInfo,
 }) => {
   const { t } = useTranslation();
+  const planFeatures = usePlanFeatures();
+  const planHighlights = usePlanHighlights();
   const [showComparison, setShowComparison] = useState(false);
 
   // Static checkout modal state
@@ -88,8 +93,8 @@ const StaticPlanSection: React.FC<StaticPlanSectionProps> = ({
       price: 0,
       currency: "£",
       period: "",
-      highlights: PLAN_HIGHLIGHTS.FREE,
-      features: PLAN_FEATURES.FREE,
+      highlights: planHighlights.FREE,
+      features: planFeatures.FREE,
       maxUsers: 5,
     },
     {
@@ -99,8 +104,8 @@ const StaticPlanSection: React.FC<StaticPlanSectionProps> = ({
       currency: "",
       period: "",
       popular: false,
-      highlights: PLAN_HIGHLIGHTS.SERVER_MONTHLY,
-      features: PLAN_FEATURES.SERVER,
+      highlights: planHighlights.SERVER_MONTHLY,
+      features: planFeatures.SERVER,
       maxUsers: "Unlimited users",
     },
     {
@@ -109,8 +114,8 @@ const StaticPlanSection: React.FC<StaticPlanSectionProps> = ({
       price: 0,
       currency: "",
       period: "",
-      highlights: PLAN_HIGHLIGHTS.ENTERPRISE_MONTHLY,
-      features: PLAN_FEATURES.ENTERPRISE,
+      highlights: planHighlights.ENTERPRISE_MONTHLY,
+      features: planFeatures.ENTERPRISE,
       maxUsers: "Custom",
     },
   ];