Prepare for open source release under GPLv3

- Add GPLv3 LICENSE file
- Add CONTRIBUTING.md with guidelines
- Update README: remove Play Store section, add license and contributing refs
- Fix feature graphic (remove grid lines, replace Waybar chip with No root)

README.md

@@ -208,22 +208,10 @@ export KEY_PASSWORD="your-key-password"
 # Output: app/build/outputs/apk/release/app-release.apk
 ```
 
-## Play Store Publishing
+## Contributing
 
-### Still needed:
-
-- [ ] **App icons** - Replace placeholder icons in `app/src/main/res/mipmap-*` with proper 512x512 adaptive icons
-- [ ] **Privacy policy** - Required URL (app uses location permission)
-- [ ] **Screenshots** - Phone screenshots for Play Store listing
-- [ ] **Feature graphic** - 1024x500 banner image
-- [ ] **Store description** - Short and full descriptions
-
-### Already configured:
-
-- [x] Release signing config
-- [x] ProGuard/R8 minification
-- [x] Target SDK 36 (meets Play Store requirements)
+See [CONTRIBUTING.md](CONTRIBUTING.md) for guidelines.
 
 ## License
 
-MIT
+GPLv3 - see [LICENSE](LICENSE)