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/src/mainview/SplashScreen.svelte

@@ -4,6 +4,7 @@
    * Auto-dismisses when the `ready` prop becomes true.
    * Fade-out transition: 300ms opacity.
    */
+  import { t } from './i18n.svelte.ts';
 
   interface Props {
     /** Set to true when app initialization is complete. */
@@ -41,7 +42,7 @@
       <span class="dot"></span>
       <span class="dot"></span>
     </div>
-    <div class="loading-label">Loading...</div>
+    <div class="loading-label">{t('splash.loading')}</div>
   </div>
 </div>