license: add GPL-3.0-or-later license

- Add full LICENSE file (GPL-3.0)
- Add SPDX headers to all 44 Kotlin source files
- Add SPDX headers to 3 iOS Swift/Info.plist files
- Update README.md and AGENTS.md with license info

README.md

@@ -160,4 +160,9 @@ The app communicates with the MyIce FastAPI backend at `https://myice.parano.ch`
 
 ## License
 
-Private project.
+Copyright (C) 2026 parano.ch
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the [GNU General Public License v3.0](LICENSE)
+as published by the Free Software Foundation, either version 3 of the
+License, or (at your option) any later version.