feat(settings): add default startup view and reader zoom preferences (#6073)

## Description of Changes

Adds two new user preferences to the General settings panel, addressing
#5908.

**Default view on launch** - a segmented control (Tools / Reader /
Automate) that controls which left-column tab is active when the app
starts. Previously the app always opened on the Tools tab with no way to
change this. Users who spend most of their time reading PDFs had to
manually switch to the Reader tab on every launch.

**Default reader zoom** - a dropdown (Auto / Fit width / Fit page /
50%–200%) that sets the initial zoom level whenever a PDF is opened in
the reader. Previously the app always applied an automatic
fit-to-viewport calculation.

Both settings are non-breaking. The defaults (`Tools` and `Auto`)
reproduce the existing behaviour exactly, so existing users see no
difference until they change a preference.

### What changed
- `preferencesService.ts` - added `StartupView` and `ViewerZoomSetting`
types plus the two new fields to `UserPreferences` with safe defaults
- `ToolWorkflowContext.tsx` - one-time startup effect that navigates to
the preferred tab on first render (mirrors the existing
`defaultToolPanelMode` sync pattern)
- `ZoomAPIBridge.tsx` - respects the zoom preference before falling back
to auto-zoom logic when a document loads
- `GeneralSection.tsx` - two new controls added below "Default tool
picker mode"; the Select uses `comboboxProps={{ withinPortal: true }}`
so the dropdown renders above the settings modal
- `en-GB/translation.toml` - new keys for labels, descriptions, and
option values

Closes #5908 

---

## Checklist

### General

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [x] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [x] I have performed a self-review of my own code
- [x] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [x] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [x] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)
<img width="1023" height="747" alt="Screenshot 2026-04-05 185718"
src="https://github.com/user-attachments/assets/6a8bc35a-d813-4ab8-b303-55bdce747a6a"
/>
<img width="1026" height="755" alt="Screenshot 2026-04-05 185620"
src="https://github.com/user-attachments/assets/d2c45134-ed32-4332-a193-1a96837ba2a3"
/>


### Testing (if applicable)

- [x] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

frontend/src/core/components/shared/config/configSections/GeneralSection.tsx

@@ -7,6 +7,7 @@ import {
   Tooltip,
   NumberInput,
   SegmentedControl,
+  Select,
   Code,
   Group,
   Anchor,
@@ -19,6 +20,8 @@ import { useTranslation } from "react-i18next";
 import { usePreferences } from "@app/contexts/PreferencesContext";
 import { useAppConfig } from "@app/contexts/AppConfigContext";
 import type { ToolPanelMode } from "@app/constants/toolPanel";
+import type { StartupView, ViewerZoomSetting } from "@app/services/preferencesService";
+import { Z_INDEX_OVER_CONFIG_MODAL } from "@app/styles/zIndex";
 import LocalIcon from "@app/components/shared/LocalIcon";
 import { updateService, UpdateSummary } from "@app/services/updateService";
 import UpdateModal from "@app/components/shared/UpdateModal";
@@ -293,6 +296,61 @@ const GeneralSection: React.FC<GeneralSectionProps> = ({ hideTitle = false, hide
               ]}
             />
           </div>
+          <div style={{ display: "flex", alignItems: "center", justifyContent: "space-between" }}>
+            <div>
+              <Text fw={500} size="sm">
+                {t("settings.general.defaultStartupView", "Default view on launch")}
+              </Text>
+              <Text size="xs" c="dimmed" mt={4}>
+                {t(
+                  "settings.general.defaultStartupViewDescription",
+                  "Choose which tab is active in the left column when the app starts",
+                )}
+              </Text>
+            </div>
+            <SegmentedControl
+              value={preferences.defaultStartupView}
+              onChange={(val: string) => updatePreference("defaultStartupView", val as StartupView)}
+              data={[
+                { label: t("settings.general.startupView.tools", "Tools"), value: "tools" },
+                { label: t("settings.general.startupView.read", "Reader"), value: "read" },
+                { label: t("settings.general.startupView.automate", "Automate"), value: "automate" },
+              ]}
+            />
+          </div>
+          <div style={{ display: "flex", alignItems: "center", justifyContent: "space-between" }}>
+            <div>
+              <Text fw={500} size="sm">
+                {t("settings.general.defaultViewerZoom", "Default reader zoom")}
+              </Text>
+              <Text size="xs" c="dimmed" mt={4}>
+                {t(
+                  "settings.general.defaultViewerZoomDescription",
+                  "Set the default zoom level when opening PDFs in the reader",
+                )}
+              </Text>
+            </div>
+            <Select
+              value={preferences.defaultViewerZoom}
+              onChange={(val: string | null) => {
+                if (val) updatePreference("defaultViewerZoom", val as ViewerZoomSetting);
+              }}
+              data={[
+                { label: t("settings.general.zoomLevel.auto", "Auto"), value: "auto" },
+                { label: t("settings.general.zoomLevel.fitWidth", "Fit width"), value: "fitWidth" },
+                { label: t("settings.general.zoomLevel.fitPage", "Fit page"), value: "fitPage" },
+                { label: "50%", value: "50" },
+                { label: "75%", value: "75" },
+                { label: "100%", value: "100" },
+                { label: "125%", value: "125" },
+                { label: "150%", value: "150" },
+                { label: "200%", value: "200" },
+              ]}
+              style={{ width: 140 }}
+              allowDeselect={false}
+              comboboxProps={{ withinPortal: true, zIndex: Z_INDEX_OVER_CONFIG_MODAL }}
+            />
+          </div>
           <div style={{ display: "flex", alignItems: "center", justifyContent: "space-between" }}>
             <div>
               <Text fw={500} size="sm">