refactor(context-mobile): migrate from Supabase to backend API + mana-core-auth

Complete migration of Context mobile app from direct Supabase access
to NestJS backend API with mana-core-auth authentication.

New files:
- context/AuthProvider.tsx: mana-core-auth integration via @manacore/shared-auth
- services/backendApi.ts: Backend API client for spaces, documents, AI, tokens

Rewritten services (same exports, backend implementation):
- supabaseService.ts: Now thin wrapper around backendApi
- aiService.ts: Uses backendApi for auth token
- tokenCountingService.ts: Model prices from backend API
- tokenTransactionService.ts: All token ops via backend API
- revenueCatService.ts: Token balance via backend API

Updated 16 consumer files (auth forms, token components, AI toolbars)

Deleted:
- utils/supabase.ts, context/AuthContext.tsx
- services/spaceService.ts, services/spaceServiceDirect.ts

Dependencies:
- Added: @manacore/shared-auth, expo-secure-store
- Removed: @supabase/supabase-js, @google/generative-ai, openai, @azure/openai

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/context/apps/mobile/package.json

@@ -18,13 +18,11 @@
 		"web": "expo start --web"
 	},
 	"dependencies": {
-		"@azure/openai": "^2.0.0",
 		"@expo/vector-icons": "^14.0.0",
-		"@google/generative-ai": "^0.24.0",
+		"@manacore/shared-auth": "workspace:*",
 		"@react-native-async-storage/async-storage": "^1.23.1",
 		"@react-native-picker/picker": "^2.11.0",
 		"@react-navigation/native": "^7.0.3",
-		"@supabase/supabase-js": "^2.38.4",
 		"expo": "^52.0.46",
 		"expo-constants": "~17.0.8",
 		"expo-dev-client": "~5.0.4",
@@ -35,10 +33,10 @@
 		"expo-router": "~4.0.6",
 		"expo-status-bar": "~2.0.1",
 		"expo-system-ui": "~4.0.9",
+		"expo-secure-store": "~14.0.1",
 		"expo-web-browser": "~14.0.2",
 		"i18next": "^25.3.2",
 		"nativewind": "latest",
-		"openai": "^4.95.0",
 		"react": "18.3.1",
 		"react-dom": "18.3.1",
 		"react-i18next": "^15.6.0",