feat(android): implement full Android frontend

- Build config: build.gradle (app + root), settings.gradle, AndroidManifest
- Application entry: PantreeApplication (Hilt), MainActivity (Compose host)
- Data layer:
  - ApiService (Retrofit interface, all 20 endpoints)
  - ApiModels (all request/response DTOs)
  - AuthInterceptor (JWT Bearer header injection)
  - TokenStore (EncryptedSharedPreferences)
  - SyncPreferences (DataStore, last-sync timestamp)
  - PantreeDatabase (Room, 5 entities)
  - Entities: PantryItemEntity, RecipeEntity, RecipeIngredientEntity, ShoppingListEntity, ShoppingListItemEntity
  - DAOs: PantryDao, RecipeDao, ShoppingListDao
  - Repositories: AuthRepository, PantryRepository, RecipeRepository, ShoppingListRepository, SyncRepository
- DI: AppModule (Hilt, provides OkHttp/Retrofit/Room/DAOs)
- Util: Result<T> sealed class, safeApiCall, toUserMessage (human-readable error mapping)
- Sync: SyncManager (DefaultLifecycleObserver, triggers on app open/foreground)
- Theme: Color, Type, Theme (Material 3, light + dark)
- Navigation: Screen sealed class, PantreeNavGraph (deep links for password reset)
- Shared components: LoadingState, ErrorState, EmptyState, OfflineBanner, PantreeTopBar
- Auth screens: SplashScreen, SignInScreen, SignUpScreen, ForgotPasswordScreen, ResetPasswordScreen + AuthViewModel
- Pantry screen: PantryScreen (list/add/edit/delete dialogs) + PantryViewModel
- Recipe screens: RecipesScreen (filter chips, search, availability badges) + RecipeDetailScreen (scale 1x/2x/3x, ingredient pantry status) + RecipeViewModel
- Shopping screens: ShoppingListsScreen + ShoppingListDetailScreen (check-off, progress bar, merge feedback, add-from-recipes) + ShoppingListViewModel
- Settings screen: sign out + account deletion (15-day window copy) + SettingsViewModel
- Tests: ResultTest (unit), RecipeDetailScreenTest (unit), PantryScreenTest (instrumented)
- README: architecture, module structure, UI states table, sync strategy, setup

Every screen handles loading / error / empty / success states.
Offline: cached Room data shown read-only.

android/README.md

@@ -0,0 +1,76 @@
+# Pantree Android
+
+Android client for Pantree — the app that tells you what you can cook with what you already have.
+
+## Architecture
+
+- **MVVM** — ViewModel + StateFlow, no LiveData
+- **Hilt** — dependency injection
+- **Room** — local cache (pantry items, recipes, shopping lists)
+- **Retrofit + OkHttp** — network layer with JWT interceptor
+- **Jetpack Compose** — declarative UI, Material 3
+- **EncryptedSharedPreferences** — JWT stored at rest
+- **DataStore** — last-sync timestamp
+
+## Module Structure
+
+```
+app/src/main/java/com/pantree/app/
+├── data/
+│   ├── local/          # Room DB, DAOs, entities, TokenStore, SyncPreferences
+│   ├── model/          # API DTOs (request/response)
+│   ├── remote/         # ApiService (Retrofit), AuthInterceptor
+│   └── repository/     # AuthRepository, PantryRepository, RecipeRepository,
+│                       # ShoppingListRepository, SyncRepository
+├── di/                 # Hilt AppModule
+├── sync/               # SyncManager (lifecycle observer)
+├── ui/
+│   ├── components/     # LoadingState, ErrorState, EmptyState, OfflineBanner, PantreeTopBar
+│   ├── navigation/     # Screen sealed class, PantreeNavGraph
+│   ├── screens/
+│   │   ├── auth/       # SplashScreen, SignInScreen, SignUpScreen,
+│   │   │               # ForgotPasswordScreen, ResetPasswordScreen + AuthViewModel
+│   │   ├── pantry/     # PantryScreen + PantryViewModel
+│   │   ├── recipe/     # RecipesScreen, RecipeDetailScreen + RecipeViewModel
+│   │   ├── shopping/   # ShoppingListsScreen, ShoppingListDetailScreen + ShoppingListViewModel
+│   │   └── settings/   # SettingsScreen + SettingsViewModel
+│   └── theme/          # Color, Type, Theme
+└── util/               # Result<T>, safeApiCall, toUserMessage
+```
+
+## UI States
+
+Every screen handles all four states:
+
+| State | Implementation |
+|-------|---------------|
+| **Loading** | `LoadingState` composable — spinner + contextual message |
+| **Error** | `ErrorState` composable — emoji + message + retry button |
+| **Empty** | `EmptyState` composable — emoji + title + subtitle + optional CTA |
+| **Success** | Full content with `LazyColumn` / detail view |
+
+Offline: cached data shown read-only, `OfflineBanner` displayed.
+
+## Sync Strategy
+
+- `SyncManager` implements `DefaultLifecycleObserver` — triggers on `onStart` (app open + foreground)
+- Delta sync via `GET /sync?since=<last_timestamp>`
+- Server timestamp stored in DataStore, used as `since` on next sync
+- Room cache updated; UI observes `Flow<List<Entity>>` — updates automatically
+
+## Setup
+
+1. Set `BASE_URL` in `app/build.gradle` debug/release buildConfigFields
+2. Set `GOOGLE_WEB_CLIENT_ID` for Google Sign-In
+3. Run backend (`npm run dev` from repo root)
+4. `./gradlew assembleDebug`
+
+## Running Tests
+
+```bash
+# Unit tests
+./gradlew test
+
+# Instrumented tests (requires emulator/device)
+./gradlew connectedAndroidTest
+```