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/editor-window/editor-window.ts

@@ -10,6 +10,7 @@ import {
   viewChild,
 } from '@angular/core';
 import { NgIcon } from '@ng-icons/core';
+import { I18nService } from '../i18n.service';
 import { ALT_LABEL, blockNativeContextMenu, EDITOR_CONTEXT, IS_MAC_CHROME, MOD_LABEL } from '../platform';
 import { RpcService } from '../rpc.service';
 import { SettingsService } from '../settings.service';
@@ -38,6 +39,7 @@ const DRAFT_DEBOUNCE_MS = 400;
   },
 })
 export class EditorWindow {
+  readonly i18n = inject(I18nService);
   readonly ctx = EDITOR_CONTEXT;
   readonly isMacChrome = IS_MAC_CHROME;
   readonly modLabel = MOD_LABEL;
@@ -273,7 +275,7 @@ export class EditorWindow {
       }
       this.savedContent.set(content);
       this.lastSentDirty = false; // bun record already cleaned by the handler
-      this.toast.success(`Saved ${this.ctx!.name}`);
+      this.toast.success(this.i18n.t('toasts.saved', { name: this.ctx!.name }));
       return true;
     } finally {
       this.saving.set(false);