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

backend/certs/pub_key.pem

@@ -0,0 +1,9 @@
+-----BEGIN PUBLIC KEY-----
+MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAuzK+YibPERKTi8CaO+Yq
+meTdJmaTqHTS/XBbnRmZgQmudwPIfybp1E+wPKixcBrw/qaj6Ewfoo+6eh++aUVw
+ImiQcmYrF+4IgpaTJMfYwYgyeig8XlGlwNNngWPZ7g+7Q7FZUjMYJ4ITq2IHCyWR
+IhQfPY9auZdBAndtcX9flDdUAzur2EDYVNYMlFMxt2wGnBGfXitsZefWdvRq5IA1
+N8zCxfCRvN9GFBjtjQXKeUmPwxLmcqKuA3fRz/THjq0oeggQK9PNQmtZgUD8nKE7
+cils+CFpkLK+a34iFIA2QHygL6itReyV7O47MD4gYYW+aCmp/L0FUcdISJtaCZ2T
+9QIDAQAB
+-----END PUBLIC KEY-----