feat: add EAS build configuration for local APK builds

- Add eas.json with development, preview, and production profiles
- Configure preview profile for APK builds (arm64-v8a only)
- Add build:apk npm script for local builds
- Update app.json with app name and Android package
- Add expo-build-properties dependency
- Update CLAUDE.md with build documentation

CLAUDE.md

@@ -23,6 +23,7 @@ npm run android -w @caldav/client      # Start on Android
 npm run ios -w @caldav/client          # Start on iOS
 npm run web -w @caldav/client          # Start web version
 npm run lint -w @caldav/client         # Run ESLint
+npm run build:apk -w @caldav/client    # Build APK locally with EAS
 ```
 
 ### Server (apps/server) - Express.js backend
@@ -42,6 +43,7 @@ npm run start -w @caldav/server        # Run compiled server (port 3000)
 | | NativeWind | Tailwind CSS for React Native |
 | | Zustand | State management |
 | | FlashList | High-performance lists |
+| | EAS Build | Local APK/IPA builds |
 | Backend | Express.js | Web framework |
 | | MongoDB | Database |
 | | Mongoose | ODM |
@@ -404,6 +406,29 @@ NODE_ENV=development        # development = pretty logs, production = JSON
 - `ChatStore`: Zustand store with addMessage(), addMessages(), updateMessage(), clearMessages() - loads from server on mount and persists across tab switches
 - Event Detail and Note screens exist as skeletons
 
+## Building
+
+### Local APK Build with EAS
+```bash
+npm run build:apk -w @caldav/client
+```
+
+This uses the `preview` profile from `eas.json` which builds an APK with:
+- `arm64-v8a` architecture only (smaller APK size)
+- No credentials required (`withoutCredentials: true`)
+- Internal distribution
+
+**Requirements:** Android SDK and Java must be installed locally.
+
+**EAS Configuration:** `apps/client/eas.json` contains build profiles:
+- `development`: Development client with internal distribution
+- `preview`: APK build for testing (used by `build:apk`)
+- `production`: Production build with auto-increment versioning
+
+**App Identity:**
+- Package name: `com.gilmour109.calchat`
+- EAS Project ID: `b722dde6-7d89-48ff-9095-e007e7c7da87`
+
 ## Documentation
 
 Detailed architecture diagrams are in `docs/`: