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>

src/bun/index.ts

@@ -8,6 +8,7 @@ import { EditorWindows } from "./editor-windows";
 import { SessionRegistry } from "./fs/registry";
 import { LocalDirWatcher } from "./fs/watcher";
 import { ManagerWindow } from "./manager-window";
+import { setLocale as setBackendLocale } from "./i18n";
 import { applyApplicationMenu } from "./menu";
 import { createAppRPC } from "./rpc-handlers";
 import { ConnectionStore } from "./store/connections";
@@ -64,6 +65,7 @@ const url = await getMainViewUrl();
 // window's preload so the first paint is already themed (no flash). Kept
 // fresh via the saveSettings flow below.
 let currentSettings = await settings.load();
+setBackendLocale(currentSettings.locale); // native menus/errors follow the UI language
 
 const isMac = process.platform === "darwin";
 
@@ -115,7 +117,12 @@ const manager = new ManagerWindow({
 
 events.openEditor = (ctx) => editors.open(ctx);
 events.settingsChanged = (s) => {
+	const localeChanged = s.locale !== currentSettings.locale;
 	currentSettings = s; // future windows' preloads bake the fresh theme
+	if (localeChanged) {
+		setBackendLocale(s.locale);
+		void applyApplicationMenu(store); // relabel the native menu in the new language
+	}
 	editors.broadcastSettings(s);
 	manager.broadcastSettings(s);
 };