feat: Convert Android app to Kotlin with modern storage API

[tstapler]

This PR converts the Android app from Java to Kotlin and updates it to use the modern storage API.

Changes Made

Android App Modernization

• Convert Java to Kotlin: Migrate MainActivity.java, FolderPicker.java, FsWatcher.java to Kotlin
• Modern Storage API: Update to use com.google.modernstorage for better file handling
• Kotlin Best Practices: Apply Kotlin idioms and null safety features
• Build Configuration: Update Gradle files for Kotlin support

CI Workflow Improvements

• Fork Compatibility: Workflows now work without requiring main repo secrets
• Unsigned Builds: Generate unsigned APKs when keystore not available
• Platform Support: Linux and macOS builds (removed Windows)
• Trigger Updates: Only run on pull requests and manual dispatch

Testing Status

• ✅ Android build compiles successfully
• ✅ Kotlin conversion maintains functionality
• ✅ CI workflows validated with actionlint
• ✅ Fork compatibility confirmed

Artifacts

The workflows will produce:

• Unsigned Android APK builds
• Linux and macOS desktop application builds

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