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/app/models/__init__.py

@@ -11,6 +11,7 @@ from .preference import PreferenceAnalysis, MarketingEffect
 from .device import Device
 from .followup import FollowupStrategy, FollowupLog
 from .system_config import SystemConfig
+from .translation_quota import TranslationQuota
 
 __all__ = [
     "User", "Product",
@@ -26,4 +27,5 @@ __all__ = [
     "Device",
     "FollowupStrategy", "FollowupLog",
     "SystemConfig",
+    "TranslationQuota",
 ]