Set up Capacitor for Android and iOS builds

Set up Capacitor to wrap the web app as native Android/iOS apps.

**Steps:**
- Install `@capacitor/core` and `@capacitor/cli`
- Init with app ID `com.jesseweigel.battlemath`
- Add android and ios platforms
- Configure `capacitor.config.ts`
- Sync build output
- Test in Android Studio emulator

This enables app store distribution.

**Acceptance criteria:**
- [ ] Capacitor configured
- [ ] Android project generated
- [ ] Build syncs correctly
- [ ] App runs in Android emulator