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

AGENTS.md

@@ -74,6 +74,10 @@ Endpoints: `/login`, `/userinfo`, `/accounts`, `/schedule?account=`, `/game/{id}
 - `keystore.properties` and `*.keystore` are gitignored — never commit
 - OAuth tokens stored via multiplatform-settings (plaintext, same as Flutter app)
 
+## License
+
+GPL-3.0-or-later. See [LICENSE](LICENSE). All source files include SPDX headers.
+
 ## Known Limitations
 
 - iOS `.xcodeproj` must be created manually in Xcode (Swift files exist in `iosApp/`)