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/shared/types.ts

@@ -62,6 +62,9 @@ export interface ConnectionsFile {
 	connections: SavedConnection[];
 }
 
+/** UI languages the app ships translations for. */
+export type SupportedLocale = "en" | "es" | "eu" | "fr" | "de" | "zh" | "ja";
+
 export type AccentTone =
 	| "graphite"
 	| "blue"
@@ -73,6 +76,8 @@ export type AccentTone =
 	| "teal";
 
 export interface AppSettings {
+	/** UI language; "system" resolves to the OS locale (fallback English). */
+	locale: "system" | SupportedLocale;
 	theme: "system" | "light" | "dark";
 	/** Dark appearance: grey ("dark") or near-black AMOLED ("darker"). */
 	darkTone: "dark" | "darker";
@@ -102,6 +107,7 @@ export interface AppSettings {
 }
 
 export const DEFAULT_SETTINGS: AppSettings = {
+	locale: "system",
 	theme: "system",
 	darkTone: "dark",
 	accent: "blue",