Phase 3.4: client_app self-managed auth cutover

Rips firebase_auth; auth talks directly to the new backend endpoints.
Anonymous-first + phone OTP work end-to-end; Google/Apple SDKs are kept
but buttons are hidden behind ENABLE_SOCIAL_AUTH until backend OAuth
credentials are provisioned.

Smoke-tested against the backend via curl:
- anonymous → PATCH display_name → /me
- OTP request (read stub code from backend log) → verify with
  anonymous_customer_id → same customer row preserved, display_name
  preserved, phone added → upgrade confirmed
- refresh rotation + logout → post-logout refresh correctly fails
  REFRESH_INVALID
- Debug APK builds clean

- pubspec: drop firebase_auth; add flutter_secure_storage
- core/auth/auth_bridge.dart: shared mutable state (access token +
  refresh callback + in-flight de-dup) — keepAlive provider
- core/auth/token_storage.dart: flutter_secure_storage wrapper
  (customer_refresh_token key)
- core/auth/social_auth_enabled.dart: const flag from
  --dart-define=ENABLE_SOCIAL_AUTH (default false)
- core/auth/auth_notifier.dart: bootstrap via stored refresh; anonymous
  via /api/shared/auth/anonymous + PATCH display_name; phone OTP via
  /api/client/auth/*; Google + Apple wired (passes anonymous_customer_id
  for upgrade); anonymity config check for ForceRegister state; granular
  error-code mapping
- core/api/api_client.dart: Bearer from bridge + postRaw(skipAuth) for
  auth endpoints + single-retry 401 refresh
- core/chat/chat_notifier.dart + core/pairing/pairing_notifier.dart: WS
  auth frame reads bridge.accessToken
- features/auth/screens/otp_screen.dart: verificationId → otpRequestId
- features/auth/screens/register_screen.dart + force_register_screen.dart:
  Google/Apple buttons gated behind kSocialAuthEnabled; force_register
  drops obsolete linkAccount() (upgrade happens server-side now via
  anonymous_customer_id)
- client_app/CLAUDE.md: Auth section rewritten (was stale on Firebase)

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

client_app/pubspec.yaml

@@ -11,12 +11,12 @@ dependencies:
   flutter:
     sdk: flutter
 
-  # Firebase
+  # Firebase (Messaging only — Auth dropped in Phase 3.4, self-managed JWT now)
   firebase_core: ^3.12.1
-  firebase_auth: ^5.5.1
   firebase_messaging: ^15.2.5
 
-  # Social login
+  # Social login (kept — activated when OAuth creds arrive; buttons hidden behind
+  # ENABLE_SOCIAL_AUTH dart-define flag until then)
   google_sign_in: ^6.2.1
   sign_in_with_apple: ^6.1.0
 
@@ -31,7 +31,8 @@ dependencies:
   flutter_hooks: ^0.20.5
 
   # Storage
-  shared_preferences: ^2.2.3
+  shared_preferences: ^2.2.3      # onboarding flag, non-sensitive
+  flutter_secure_storage: ^9.2.2  # refresh token (encrypted)
 
   # Navigation
   go_router: ^13.2.1