feat: Add automatic Android asset validation to prevent build failures

- Add validate_android_assets() function to build-android.sh
- Check for missing source assets (icon.png, splash.png, splash_dark.png)
- Verify Android resources exist (drawable/splash.png, mipmap/*/ic_launcher*.png)
- Auto-regenerate missing resources using @capacitor/assets
- Integrate validation into main build process with exit code 9
- Add npm run assets:validate:android for manual validation
- Support --assets-only flag for asset-only operations
- Create comprehensive documentation in doc/android-asset-validation.md

Fixes build failures caused by missing drawable/splash and mipmap/ic_launcher resources.
Prevents "Android resource linking failed" errors during Gradle builds.

Resolves: Android build failures due to missing asset resources

package.json

@@ -31,6 +31,7 @@
     "build:native": "vite build && npx cap sync && npx capacitor-assets generate",
     "assets:config": "npx tsx scripts/assets-config.ts",
     "assets:validate": "npx tsx scripts/assets-validator.ts",
+    "assets:validate:android": "./scripts/build-android.sh --assets-only",
     "assets:clean": "rimraf android/app/src/main/res/mipmap-* ios/App/App/Assets.xcassets/**/AppIcon*.png ios/App/App/Assets.xcassets/**/Splash*.png || true",
     "build:ios": "./scripts/build-ios.sh",
     "build:ios:dev": "./scripts/build-ios.sh --dev",