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/ChatInput.svelte

@@ -7,6 +7,7 @@
     provider?: string;
     contextPct?: number;
     disabled?: boolean;
+    placeholder?: string;
     onSend?: () => void;
     onInput?: (v: string) => void;
   }
@@ -17,6 +18,7 @@
     provider = 'claude',
     contextPct = 78,
     disabled = false,
+    placeholder = 'Ask Claude anything...',
     onSend,
     onInput,
   }: Props = $props();
@@ -94,7 +96,7 @@
   <textarea
     bind:this={textareaEl}
     class="chat-textarea"
-    placeholder="Ask Claude anything..."
+    {placeholder}
     rows="1"
     {value}
     oninput={handleInput}