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/components/AgentMarkdown.test.tsx

@@ -0,0 +1,11 @@
+import { describe, expect, it } from 'vitest'
+import { looksLikeMarkdown } from '../../src/components/ai/AgentMarkdown'
+
+describe('looksLikeMarkdown', () => {
+  it('detects tables, headings and lists', () => {
+    expect(looksLikeMarkdown('| a | b |\n| - | - |\n| 1 | 2 |')).toBe(true)
+    expect(looksLikeMarkdown('## 总结\n完成')).toBe(true)
+    expect(looksLikeMarkdown('- nginx\n- sshd')).toBe(true)
+    expect(looksLikeMarkdown('负载正常。')).toBe(false)
+  })
+})