fix(ui): Wire system back gesture to in-app navigation

Add PlatformBackHandler (expect/actual) to intercept the back gesture
on Android via BackHandler, and enable OnBackInvokedCallback in the
manifest so the swipe-back gesture triggers onBack instead of exiting
the app.

Fixes the back gesture exiting to home from the event detail screen.

composeApp/src/commonMain/kotlin/ch/parano/myice/ui/PlatformBackHandler.kt

@@ -0,0 +1,23 @@
+// 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.ui
+
+import androidx.compose.runtime.Composable
+
+@Composable
+expect fun PlatformBackHandler(enabled: Boolean = true, onBack: () -> Unit)

composeApp/src/commonMain/kotlin/ch/parano/myice/ui/screens/EventDetailScreen.kt

@@ -43,6 +43,7 @@ import androidx.compose.ui.Modifier
 import androidx.compose.ui.graphics.Color
 import androidx.compose.ui.text.font.FontWeight
 import androidx.compose.ui.unit.dp
+import ch.parano.myice.ui.PlatformBackHandler
 import ch.parano.myice.ui.components.LoadingIndicator
 import ch.parano.myice.util.formatEventDate
 import ch.parano.myice.util.formatEventTimeRange
@@ -60,6 +61,8 @@ fun EventDetailScreen(
     onBack: () -> Unit,
     modifier: Modifier = Modifier,
 ) {
+    PlatformBackHandler { onBack() }
+
     LaunchedEffect(gameId, account) {
         viewModel.loadEventDetail(gameId, account)
     }