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/core/tools/formFill/FormFieldSidebar.tsx

@@ -34,7 +34,7 @@ interface FormFieldSidebarProps {
 }
 
 export function FormFieldSidebar({ visible, onToggle }: FormFieldSidebarProps) {
-  useTranslation();
+  const { t } = useTranslation();
   const { state, setValue, setActiveField } = useFormFill();
   const { fields, activeFieldName, loading } = state;
   const activeFieldRef = useRef<HTMLDivElement>(null);
@@ -119,7 +119,7 @@ export function FormFieldSidebar({ visible, onToggle }: FormFieldSidebarProps) {
           variant="subtle"
           size="sm"
           onClick={onToggle}
-          aria-label="Close sidebar"
+          aria-label={t("formFill.sidebar.close", "Close sidebar")}
         >
           <CloseIcon sx={{ fontSize: 16 }} />
         </ActionIcon>

frontend/editor/src/core/tools/formFill/FormFill.tsx

@@ -27,6 +27,7 @@ import {
   Tooltip,
   ActionIcon,
 } from "@mantine/core";
+import { useTranslation } from "react-i18next";
 import {
   useFormFill,
   useAllFormValues,
@@ -120,6 +121,7 @@ const _MODE_TABS: ModeTabDef[] = [
 // ---------------------------------------------------------------------------
 
 const FormFill = (_props: BaseToolProps) => {
+  const { t } = useTranslation();
   const { selectedTool } = useNavigation();
   const { selectors, state: fileState } = useFileState();
 
@@ -488,7 +490,10 @@ const FormFill = (_props: BaseToolProps) => {
 
                 {/* Flatten toggle */}
                 <Switch
-                  label="Flatten after filling"
+                  label={t(
+                    "formFill.flattenAfterFilling",
+                    "Flatten after filling",
+                  )}
                   checked={flatten}
                   onChange={(e) => setFlatten(e.currentTarget.checked)}
                   size="xs"
@@ -507,15 +512,22 @@ const FormFill = (_props: BaseToolProps) => {
                       loading={saving}
                       disabled={!formState.isDirty && !flattenChanged}
                     >
-                      Save
+                      {t("formFill.save", "Save")}
                     </Button>
 
-                    <Tooltip label="Re-scan fields" withArrow position="bottom">
+                    <Tooltip
+                      label={t("formFill.rescanFields", "Re-scan fields")}
+                      withArrow
+                      position="bottom"
+                    >
                       <ActionIcon
                         variant="light"
                         size="md"
                         onClick={handleRefresh}
-                        aria-label="Re-scan form fields"
+                        aria-label={t(
+                          "formFill.rescanFormFields",
+                          "Re-scan form fields",
+                        )}
                       >
                         <RefreshIcon sx={{ fontSize: 16 }} />
                       </ActionIcon>