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

composeApp/src/commonMain/kotlin/ch/parano/myice/models/Event.kt

@@ -1,3 +1,20 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+// MyIce Kotlin Multiplatform — schedule/convocation viewer
+// 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 as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
 package ch.parano.myice.models
 
 import kotlinx.serialization.SerialName