feat(i18n): add multi-language support (ja/ko/vi/ru/zh-TW) with system auto-detection

- Extend AppLanguage to 7 locales: zh-CN, zh-TW, en, ja, ko, vi, ru
- Auto-detect system language via navigator.languages; fallback to English
- Add BCP47 locale mapping for date formatting and sorting
- Create 50 translation JSON files (5 languages × 10 namespaces, 1137 keys each)
- Update language selector with native language names
- Extend locale key parity test to cover all 7 languages (70 assertions)
- Add VNC/RDP desktop, serial terminal, AI agent, port forwarding, and SFTP features
- Add credential encryption, host metrics, session backup crypto, and zmodem bridge
- Update Rust backend: desktop services, host metrics, SSH/SFTP enhancements

test/stores/settingsMigration.test.ts

@@ -1,12 +1,17 @@
-import { describe, expect, it } from 'vitest'
+import { beforeEach, describe, expect, it } from 'vitest'
 import { normalizeAppSettings } from '../../src/stores/settingsStore'
+import { isAppLanguage } from '../../src/i18n/detectLocale'
 
 describe('settings migration', () => {
-  it('drops obsolete legacy settings while preserving supported values', () => {
+  beforeEach(() => {
+    // language default depends on navigator; tests assert explicit values
+  })
+
+  it('keeps supported language and drops obsolete legacy settings', () => {
     const settings = normalizeAppSettings({
       theme: 'light',
       timeout: 45,
-      language: 'en-US',
+      language: 'en',
       showNotifications: true,
       encryptPasswords: false,
       lockOnSleep: true,
@@ -14,9 +19,14 @@ describe('settings migration', () => {
 
     expect(settings.theme).toBe('light')
     expect(settings.timeout).toBe(45)
-    expect(settings).not.toHaveProperty('language')
+    expect(settings.language).toBe('en')
     expect(settings).not.toHaveProperty('showNotifications')
     expect(settings).not.toHaveProperty('encryptPasswords')
     expect(settings).not.toHaveProperty('lockOnSleep')
   })
+
+  it('falls back to a valid language when stored value is invalid', () => {
+    const settings = normalizeAppSettings({ language: 'fr-FR' })
+    expect(isAppLanguage(settings.language)).toBe(true)
+  })
 })