chore(i18n): consolidate PURO auth heading keys into zh.ts

Login and Register heading strings moved from hardcoded Chinese to
auth.puroLoginTitle / puroLoginSub / puroRegisterTitle / puroRegisterSub.

Landing (LandingView) and Docs (DocsView) intentionally keep hardcoded
Chinese this cycle (see spec §6 note 5 — English version deferred).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/i18n/locales/zh.ts

@@ -498,7 +498,12 @@ export default {
     invalidResetLink: '无效的重置链接',
     invalidResetLinkHint: '此密码重置链接无效或已过期。请重新请求一个新链接。',
     requestNewResetLink: '请求新的重置链接',
-    invalidOrExpiredToken: '密码重置链接无效或已过期。请重新请求一个新链接。'
+    invalidOrExpiredToken: '密码重置链接无效或已过期。请重新请求一个新链接。',
+    // PURO AI redesign
+    puroLoginTitle: '登录',
+    puroLoginSub: '用你的 PURO AI 账户继续',
+    puroRegisterTitle: '创建账户',
+    puroRegisterSub: '5 分钟开始用 PURO AI',
   },
 
   // Dashboard