Complete client UI i18n migration

Adds client-side i18n support, language settings, translated UI copy, and locale parity coverage.

packages/app/src/hooks/use-settings/storage.ts

@@ -1,6 +1,7 @@
 import { isSyntaxThemeId, type SyntaxThemeId } from "@getpaseo/highlight";
 import type { QueryClient } from "@tanstack/react-query";
 import type { DesktopSettings } from "@/desktop/settings/desktop-settings";
+import { parseAppLanguage, type AppLanguage } from "@/i18n/locales";
 import { THEME_TO_UNISTYLES, type ThemeName } from "@/styles/theme";
 
 export const APP_SETTINGS_KEY = "@paseo:app-settings";
@@ -26,6 +27,7 @@ export const MAX_FONT_FAMILY_LENGTH = 200;
 
 export interface AppSettings {
   theme: ThemeName | "auto";
+  language: AppLanguage;
   sendBehavior: SendBehavior;
   serviceUrlBehavior: ServiceUrlBehavior;
   terminalScrollbackLines: number;
@@ -43,6 +45,7 @@ export interface Settings extends AppSettings {
 
 export const DEFAULT_CLIENT_SETTINGS: AppSettings = {
   theme: "auto",
+  language: "system",
   sendBehavior: "interrupt",
   serviceUrlBehavior: "ask",
   terminalScrollbackLines: DEFAULT_TERMINAL_SCROLLBACK_LINES,
@@ -149,6 +152,10 @@ function pickAppSettings(stored: Partial<AppSettings>): Partial<AppSettings> {
   if (typeof stored.theme === "string" && VALID_THEMES.has(stored.theme)) {
     result.theme = stored.theme;
   }
+  const language = parseAppLanguage(stored.language);
+  if (language !== null) {
+    result.language = language;
+  }
   if (stored.sendBehavior === "interrupt" || stored.sendBehavior === "queue") {
     result.sendBehavior = stored.sendBehavior;
   }