feat: 解决 CDC 去重、集中备份与首次初始化问题 (#105)

实现 CDC 内容寻址仓库、远程 Agent 中央中转备份与首次初始化体验，并补充安全校验、测试及双语文档。

Closes #94
Closes #101
Closes #104

web/src/i18n.test.ts

@@ -0,0 +1,21 @@
+import i18n, { normalizeLanguage, setApplicationLanguage } from './i18n'
+
+describe('application language', () => {
+  afterEach(async () => {
+    await setApplicationLanguage('zh-CN')
+  })
+
+  it('normalizes supported English variants', () => {
+    expect(normalizeLanguage('en')).toBe('en-US')
+    expect(normalizeLanguage('en-GB')).toBe('en-US')
+    expect(normalizeLanguage('zh-CN')).toBe('zh-CN')
+  })
+
+  it('persists the language selected before login', async () => {
+    await setApplicationLanguage('en-US')
+
+    expect(localStorage.getItem('backupx-language')).toBe('en-US')
+    expect(document.documentElement.lang).toBe('en-US')
+    expect(i18n.t('auth.setupTitle')).toBe('System setup')
+  })
+})