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/AgentPane.svelte

@@ -2,6 +2,7 @@
   import { tick } from 'svelte';
   import ChatInput from './ChatInput.svelte';
   import type { AgentMessage, AgentStatus } from './agent-store.svelte.ts';
+  import { t } from './i18n.svelte.ts';
 
   interface Props {
     messages: AgentMessage[];
@@ -68,10 +69,10 @@
   }
 
   function statusLabel(s: AgentStatus) {
-    if (s === 'running') return 'Running';
-    if (s === 'error') return 'Error';
-    if (s === 'done') return 'Done';
-    return 'Idle';
+    if (s === 'running') return t('agent.status.running');
+    if (s === 'error') return t('agent.status.error');
+    if (s === 'done') return t('agent.status.done');
+    return t('agent.status.idle');
   }
 
   function onResizeMouseDown(e: MouseEvent) {
@@ -106,7 +107,7 @@
   <span class="strip-sep" aria-hidden="true"></span>
   <span class="strip-cost">{fmtCost(costUsd)}</span>
   {#if status === 'running' && onStop}
-    <button class="strip-stop-btn" onclick={onStop} title="Stop agent" aria-label="Stop agent">
+    <button class="strip-stop-btn" onclick={onStop} title={t('agent.prompt.stop')} aria-label={t('agent.prompt.stop')}>
       <svg viewBox="0 0 16 16" fill="currentColor" aria-hidden="true">
         <rect x="3" y="3" width="10" height="10" rx="1" />
       </svg>
@@ -206,6 +207,7 @@
       {model}
       {provider}
       {contextPct}
+      placeholder={t('agent.prompt.placeholder')}
       onSend={handleSend}
       onInput={(v) => (promptText = v)}
     />