feat: WeChat Pay integration, translation quota management, login UX fixes

- WeChat Pay APIv3 integration (JSAPI + Native) with cert-based auth
- TranslationQuota model + admin management UI (配额 tab)
- Alibaba MT provider now checks quota before translation
- Fix: admin tabs scrollable on mobile, remove header-card
- Fix: profile/login navigation - logout stays on profile, login returns to profile
- Fix: login form now visible by default (no extra click to show)
- Fix: home page translate link uses navigateTo (was switchTab to non-tabBar page)
- Add .coverage and apiclient_key.pem to gitignore

.gitignore

@@ -8,6 +8,7 @@ env/
 venv/
 ENV/
 *.egg-info/
+.coverage
 dist/
 build/
 .eggs/
@@ -22,6 +23,7 @@ build/
 .env
 .env.local
 .env.*.local
+backend/certs/apiclient_key.pem
 
 # Logs
 *.log