feat(auth): migrate Flutter IAuthPlatform to Next.js 16 SDK service layer

Adds the third-party social login SDK integration layer (Google + Facebook),
mirroring the Flutter `IAuthPlatform` interface from
`lib/data/services/auth/`. The platform produces identity tokens only; callers
use the existing `authRepository.googleLogin / facebookLogin /
facebookIdLogin` (already migrated) to complete the login.

Architecture (event-bridge pattern):
  - `IAuthPlatform` is plain TypeScript (no React dependency)
  - `WebAuthPlatform` dispatches DOM `CustomEvent`s; `AuthPlatformBridge`
    ("use client" component) hosts the React SDK hooks and resolves promises
  - `MockAuthPlatform` returns hardcoded tokens when
    `NEXT_PUBLIC_USE_MOCK_AUTH=true` (mirrors Dart `kDebugMode` branch)

SDK choices:
  - `@react-oauth/google@0.13.5` for Google
  - `@greatsumini/react-facebook-login@3.4.0` (maintained fork of
    archived `react-facebook`, React 19 compatible) for Facebook
  - For Google: uses raw GIS `google.accounts.id.initialize` + `prompt` API
    since `@react-oauth/google` does not expose an `id_token` flow variant
    (existing `GoogleLoginRequest.idToken` field expects JWT id_token)

Files added:
  - src/data/services/auth/{auth_platform,web_auth_platform,
    mock_auth_platform,auth_event_bridge,auth_bridge_component,
    initialize_google_auth,initialize_facebook_auth,global.d.ts,index}.ts(x)
  - src/data/schemas/auth/facebook_user_data.ts (Zod schema)
  - src/data/dto/auth/facebook_user_data.ts (DTO class)

Files modified:
  - package.json, pnpm-lock.yaml: added 2 deps
  - .env.example: 4 new NEXT_PUBLIC_* env vars
  - barrelsby.json: registered src/data/services[/auth] directories
  - implementation_plan.md: appended third-party auth section
  - src/data/dto/{auth,chat,metrics,user}/index.ts: regenerated barrels
  - src/data/schemas/{auth,chat,metrics,user}/index.ts: regenerated barrels

Files deleted:
  - src/data/storage/index.ts: pre-existing TS2308 conflict (sync + async
    `AuthStorage` both re-exported from auto-generated barrel; no consumer
    uses the barrel — all imports use direct paths)

Scope (per user direction):
  - Client SDK integration only; no React hook/context, no UI, no Apple
  - Platform produces tokens; caller drives `authRepository` / `authApi`
  - v1 single-popup semantics (concurrent popups overwrite)
  - Bridge component is code-ready but NOT mounted in
    `src/app/layout.tsx`; next-step UI migration wires the provider chain

Verified: `npx tsc --noEmit` (0 errors), `pnpm lint` (0 errors),
`pnpm build` (Next.js 16.2.7 + Turbopack compiled successfully).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

barrelsby.json

@@ -1,17 +1,23 @@
 {
   "directory": [
-    "./src/data/models",
-    "./src/data/models/auth",
-    "./src/data/models/chat",
-    "./src/data/models/metrics",
-    "./src/data/models/user",
+    "./src/data/dto",
+    "./src/data/dto/auth",
+    "./src/data/dto/chat",
+    "./src/data/dto/metrics",
+    "./src/data/dto/user",
+    "./src/data/schemas",
+    "./src/data/schemas/auth",
+    "./src/data/schemas/chat",
+    "./src/data/schemas/metrics",
+    "./src/data/schemas/user",
     "./src/data/api",
-    "./src/data/storage",
     "./src/data/storage/auth",
     "./src/data/storage/user",
     "./src/data/storage/chat",
     "./src/data/storage/app",
     "./src/data/repositories",
+    "./src/data/services",
+    "./src/data/services/auth",
     "./src/design"
   ]
 }