Internationalization: 7 languages with live switching

Custom signal-based runtime (no i18n dependency): shared src/shared/i18n
dictionaries, ui I18nService.t() reactive by construction so language
switches live, and a backend i18n.ts for native menus + thrown errors.
Every user-facing string keyed across templates, toasts, dialogs, file
kinds, columns, macOS menu/context menus, and errors. Language picker in
Options (auto-detects OS locale, falls back to English); switching
relabels the webview and native chrome instantly. English/Spanish/French/
German fully drafted; Chinese/Japanese/Basque drafted, flagged for native
review. Docs updated.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ui/src/app/dialogs/prompt-dialog.ts

@@ -7,6 +7,7 @@ import {
   signal,
   viewChild,
 } from '@angular/core';
+import { I18nService } from '../i18n.service';
 import { DialogsService, type PromptRequest } from '../dialogs.service';
 import { FocusService } from '../focus.service';
 
@@ -34,7 +35,7 @@ import { FocusService } from '../focus.service';
             (click)="cancel()"
             class="rounded-md px-3 py-1.5 text-xs font-medium hover:bg-accent"
           >
-            Cancel
+            {{ i18n.t('common.cancel') }}
           </button>
           <button
             (click)="submit()"
@@ -49,6 +50,7 @@ import { FocusService } from '../focus.service';
   `,
 })
 export class PromptDialog {
+  readonly i18n = inject(I18nService);
   readonly request = input.required<PromptRequest>();
   readonly value = signal('');