feat(electrobun): i18n system — @formatjs/intl + Svelte 5 runes + 3 locales

- i18n.svelte.ts: store with $state locale + createIntl(), t() function,
  formatDate/Number/RelativeTime, getDir() for RTL, async setLocale()
- i18n.types.ts: TranslationKey union (codegen from en.json)
- locales/en.json: 200+ strings in ICU MessageFormat
- locales/pl.json: full Polish translation
- locales/ar.json: partial Arabic (validates 6-form plural + RTL)
- scripts/i18n-types.ts: codegen script for type-safe keys
- 6 components wired: StatusBar, AgentPane, CommandPalette,
  SettingsDrawer, SplashScreen, ChatInput
- Language selector in AppearanceSettings
- App.svelte: document.dir reactive for RTL
- CONTRIBUTING_I18N.md: guide for adding languages

Note: currently Electrobun-only. Will extract to @agor/i18n shared
package for both Tauri and Electrobun.

ui-electrobun/CONTRIBUTING_I18N.md

@@ -0,0 +1,104 @@
+# i18n Contributing Guide
+
+This project uses ICU MessageFormat via `@formatjs/intl` with a custom Svelte 5 store.
+
+## Adding a New Language
+
+1. Create `locales/<tag>.json` (e.g. `locales/de.json`) with all keys from `locales/en.json`.
+2. Register the locale in `src/mainview/i18n.svelte.ts`:
+   - Add an entry to `AVAILABLE_LOCALES` with tag, label, nativeLabel, and dir.
+   - Add a dynamic import entry to the `loaders` map.
+3. The language will automatically appear in Settings > Appearance > Language.
+
+## Key Naming Convention
+
+Use dot-notation: `component.element.state`
+
+```
+sidebar.settings        -- component.element
+agent.status.running    -- component.element.state
+settings.appearance     -- component.element
+common.cancel           -- shared across components
+errors.connectionFailed -- error messages
+```
+
+Group prefixes:
+- `sidebar.*` -- left sidebar
+- `agent.*` -- agent pane
+- `terminal.*` -- terminal tabs
+- `settings.*` -- settings drawer
+- `statusbar.*` -- bottom status bar
+- `notifications.*` -- notification drawer
+- `files.*` -- file browser
+- `search.*` -- search overlay
+- `comms.*` -- communications tab
+- `tasks.*` -- task board
+- `errors.*` -- error messages
+- `splash.*` -- splash screen
+- `common.*` -- shared labels (cancel, save, delete)
+- `project.*` -- project management
+- `palette.*` -- command palette
+
+## ICU MessageFormat Syntax
+
+### Plain text
+```json
+"common.save": "Save"
+```
+
+### Plurals (English: one/other)
+```json
+"search.resultsCount": "{count, plural, one {{count} result} other {{count} results}}"
+```
+
+### Plurals (Polish: one/few/many/other)
+```json
+"search.resultsCount": "{count, plural, one {{count} wynik} few {{count} wyniki} many {{count} wynikow} other {{count} wynikow}}"
+```
+
+### Plurals (Arabic: zero/one/two/few/many/other)
+```json
+"tasks.taskCount": "{count, plural, =0 {no tasks} one {one task} two {two tasks} few {{count} tasks} many {{count} tasks} other {{count} tasks}}"
+```
+
+### Interpolation
+```json
+"errors.unhandled": "Unhandled error: {message}"
+```
+
+### Select
+```json
+"agent.status": "{status, select, running {Running} idle {Idle} other {Unknown}}"
+```
+
+## Using Translations in Components
+
+```svelte
+<script lang="ts">
+  import { t } from './i18n.svelte.ts';
+</script>
+
+<button>{t('common.save')}</button>
+<span>{t('sidebar.notifCount', { count: 5 })}</span>
+```
+
+## Testing Translations
+
+1. Switch language in Settings > Appearance > Language.
+2. All `t()` calls update reactively -- no page reload needed.
+3. Missing keys fall back to the key name itself (e.g. `"agent.status.running"`).
+4. Check the browser console for `[i18n]` warnings about missing or malformed messages.
+
+## Regenerating Types
+
+After adding or removing keys in `locales/en.json`:
+
+```bash
+bun scripts/i18n-types.ts
+```
+
+This regenerates `src/mainview/i18n.types.ts` with the updated `TranslationKey` union type. TypeScript will then flag any `t()` calls using keys that no longer exist.
+
+## RTL Support
+
+Arabic (`ar`) sets `document.dir = 'rtl'` automatically. CSS should use logical properties (`inline-start`/`inline-end`) instead of `left`/`right` where possible.