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)

CONTRIBUTING.md

@@ -0,0 +1,38 @@
+# Contributing
+
+## Issues
+
+- Check existing issues before opening a new one
+- Include Android version, device model, and steps to reproduce for bugs
+- Feature requests welcome
+
+## Pull Requests
+
+1. Fork the repo
+2. Create a feature branch (`git checkout -b feature/my-change`)
+3. Commit your changes
+4. Push and open a PR against `main`
+
+## Code Style
+
+- Follow [Kotlin coding conventions](https://kotlinlang.org/docs/coding-conventions.html)
+- Use meaningful names, avoid abbreviations
+- Keep functions focused and small
+
+## Building
+
+```bash
+./gradlew assembleDebug
+```
+
+Or use the justfile:
+
+```bash
+just build
+```
+
+## Testing on Device
+
+```bash
+just install  # builds and installs debug APK via adb
+```