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

BUILDING.md

@@ -164,6 +164,9 @@ npm run clean:android
 npm run build:ios              # Regenerates iOS project
 npm run build:android          # Regenerates Android project
 
+# Fix Android asset issues
+npm run assets:validate:android # Validates and regenerates missing Android assets
+
 # Check environment
 npm run test:web               # Verifies web setup
 ```
@@ -172,6 +175,7 @@ npm run test:web               # Verifies web setup
 
 - **iOS**: Ensure Xcode and Command Line Tools are installed
 - **Android**: Ensure Android Studio and SDK are configured
+  - If you encounter "resource drawable/splash not found" errors, run `npm run assets:validate:android`
 - **Electron**: Ensure platform-specific build tools are installed
 
 ### Next Steps
@@ -1184,6 +1188,69 @@ npm run build:android:assets         # Generate assets only
 npm run build:android:deploy         # Build and deploy to connected device
 ```
 
+#### Android Asset Validation
+
+The Android build system now includes automatic asset validation to prevent build failures caused by missing resources. This system:
+
+- **Validates Source Assets**: Checks that required source files exist in `resources/`
+- **Checks Android Resources**: Verifies that generated Android resources are present
+- **Auto-Regenerates**: Automatically regenerates missing resources when detected
+- **Provides Clear Errors**: Gives helpful guidance when issues occur
+
+##### Asset Validation Commands
+
+```bash
+# Validate and regenerate Android assets if needed
+npm run assets:validate:android
+
+# Alternative command for asset validation
+./scripts/build-android.sh --assets-only
+
+# Check asset configuration only (no regeneration)
+npm run assets:validate
+```
+
+##### What Gets Validated
+
+**Source Assets (Required):**
+- `resources/icon.png` - App icon source
+- `resources/splash.png` - Splash screen source  
+- `resources/splash_dark.png` - Dark mode splash source
+
+**Android Resources (Generated):**
+- `android/app/src/main/res/drawable/splash.png` - Splash screen drawable
+- `android/app/src/main/res/mipmap-*/ic_launcher.png` - App icons for all densities
+- `android/app/src/main/res/mipmap-*/ic_launcher_round.png` - Round app icons for all densities
+
+##### Automatic Validation
+
+Asset validation runs automatically during all Android builds:
+
+```bash
+# All these commands now include asset validation
+npm run build:android:studio
+npm run build:android:prod
+npm run build:android:debug
+```
+
+If validation fails, the build stops with clear error messages and guidance on how to fix the issues.
+
+##### Troubleshooting Asset Issues
+
+If you encounter asset-related build failures:
+
+```bash
+# Check what's missing
+npm run assets:validate:android
+
+# Clean and regenerate everything
+npm run clean:android
+npm run assets:validate:android
+npm run build:android:studio
+```
+
+For more detailed information, see [Android Asset Validation Documentation](doc/android-asset-validation.md).
+
 #### Android Automated Build Script
 
 The recommended way to build for Android is using the automated build script: