Add unit tests for data file rotation utility functions

- Implement tests for writing data files with rotation, ensuring backups are created only on changes.
- Verify that old backups are rotated correctly and the maximum number of backups is maintained.
- Test restoration of backups while preserving the current state as a backup.
- Utilize Vitest for testing framework and manage temporary file storage during tests.

android-app/app/src/main/java/de/harheimertc/data/ApiService.kt

@@ -37,6 +37,12 @@ data class SpielplanResponse(
     val seasons: List<SeasonDto> = emptyList(),
 )
 data class SeasonDto(val slug: String = "", val label: String = "")
+data class MannschaftenSeasonsResponse(
+    val success: Boolean = false,
+    val seasons: List<SeasonDto> = emptyList(),
+    val currentSeason: String = "",
+    val defaultSeason: String = "",
+)
 data class SpielDto(
     @param:Json(name = "Termin") val termin: String = "",
     @param:Json(name = "HeimMannschaft") val heimMannschaft: String = "",
@@ -584,6 +590,9 @@ interface ApiService {
     @GET("/api/mannschaften")
     suspend fun mannschaften(@Query("season") season: String? = null): Response<ResponseBody>
 
+    @GET("/api/mannschaften/seasons")
+    suspend fun mannschaftenSeasons(): Response<MannschaftenSeasonsResponse>
+
     @GET("/api/config")
     suspend fun config(): Response<ConfigResponse>
 

android-app/app/src/main/java/de/harheimertc/repositories/MannschaftenRepository.kt

@@ -1,6 +1,8 @@
 package de.harheimertc.repositories
 
 import de.harheimertc.data.ApiService
+import de.harheimertc.data.MannschaftenSeasonsResponse
+import de.harheimertc.data.SeasonDto
 import java.util.Locale
 import javax.inject.Inject
 import javax.inject.Singleton
@@ -29,6 +31,12 @@ class MannschaftenRepository @Inject constructor(private val api: ApiService) {
         parseCsv(response.body()?.string().orEmpty())
     }
 
+    suspend fun fetchSeasons(): Result<MannschaftenSeasonsResponse> = runCatching {
+        val response = api.mannschaftenSeasons()
+        if (!response.isSuccessful) error("Saisons konnten nicht geladen werden.")
+        response.body() ?: error("Saisons konnten nicht geladen werden.")
+    }
+
     private fun parseCsv(csv: String): List<Mannschaft> = csv.lineSequence()
         .filter(String::isNotBlank)
         .drop(1)

android-app/app/src/main/java/de/harheimertc/ui/screens/mannschaften/MannschaftenScreen.kt

@@ -18,7 +18,10 @@ import androidx.compose.foundation.shape.RoundedCornerShape
 import androidx.compose.material3.Button
 import androidx.compose.material3.ButtonDefaults
 import androidx.compose.material3.CircularProgressIndicator
+import androidx.compose.material3.DropdownMenu
+import androidx.compose.material3.DropdownMenuItem
 import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.OutlinedButton
 import androidx.compose.material3.Surface
 import androidx.compose.material3.Text
 import androidx.compose.material3.TextButton
@@ -27,6 +30,7 @@ import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.collectAsState
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
 import androidx.compose.runtime.saveable.rememberSaveable
 import androidx.compose.runtime.setValue
 import androidx.compose.ui.Alignment
@@ -34,6 +38,7 @@ import androidx.compose.ui.Modifier
 import androidx.compose.ui.graphics.Brush
 import androidx.compose.ui.graphics.Color
 import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.text.style.TextAlign
 import androidx.compose.ui.unit.dp
 import androidx.hilt.navigation.compose.hiltViewModel
 import androidx.navigation.NavController
@@ -43,6 +48,7 @@ import de.harheimertc.repositories.Mannschaft
 import de.harheimertc.ui.navigation.Destinations
 import de.harheimertc.ui.theme.Accent100
 import de.harheimertc.ui.theme.Accent500
+import de.harheimertc.ui.theme.Accent700
 import de.harheimertc.ui.theme.Accent900
 import de.harheimertc.ui.theme.Primary100
 import de.harheimertc.ui.theme.Primary600
@@ -64,8 +70,17 @@ fun MannschaftenScreen(
             BackLink(navController, showBackNavigation)
             Text("Unsere Mannschaften", style = MaterialTheme.typography.displayLarge, color = Accent900, modifier = Modifier.padding(top = 18.dp))
             Text("Unsere aktiven Mannschaften in der aktuellen Saison", color = Accent500, modifier = Modifier.padding(top = 8.dp))
+            if (state.seasons.isNotEmpty()) {
+                SeasonSelector(
+                    seasons = state.seasons,
+                    selectedSeason = state.selectedSeason,
+                    onSeasonSelected = viewModel::selectSeason,
+                    modifier = Modifier.padding(top = 14.dp),
+                )
+            }
         }
         when {
+            state.seasonsLoading -> item { Loading() }
             state.loading -> item { Loading() }
             state.error != null -> item { ErrorPanel(state.error.orEmpty(), viewModel::load) }
             state.teams.isEmpty() -> item { Text("Keine Mannschaftsdaten geladen", color = Accent500) }
@@ -88,6 +103,38 @@ fun MannschaftenScreen(
     }
 }
 
+@Composable
+private fun SeasonSelector(
+    seasons: List<de.harheimertc.data.SeasonDto>,
+    selectedSeason: String,
+    onSeasonSelected: (String) -> Unit,
+    modifier: Modifier = Modifier,
+) {
+    var open by remember { mutableStateOf(false) }
+    val selectedLabel = seasons.firstOrNull { it.slug == selectedSeason }?.label ?: selectedSeason
+
+    Column(modifier = modifier.fillMaxWidth(), verticalArrangement = Arrangement.spacedBy(8.dp)) {
+        Text("Saison", color = Accent700, style = MaterialTheme.typography.labelSmall)
+        BoxWithConstraints {
+            OutlinedButton(onClick = { open = true }, modifier = Modifier.fillMaxWidth()) {
+                Text(selectedLabel.ifBlank { "-" }, modifier = Modifier.weight(1f), textAlign = TextAlign.Start)
+                Text("v")
+            }
+            DropdownMenu(expanded = open, onDismissRequest = { open = false }) {
+                seasons.forEach { season ->
+                    DropdownMenuItem(
+                        text = { Text(season.label.ifBlank { season.slug }) },
+                        onClick = {
+                            open = false
+                            onSeasonSelected(season.slug)
+                        },
+                    )
+                }
+            }
+        }
+    }
+}
+
 @Composable
 private fun TeamCard(team: Mannschaft, onOpen: () -> Unit) {
     Surface(

android-app/app/src/main/java/de/harheimertc/ui/screens/mannschaften/MannschaftenViewModel.kt

@@ -5,6 +5,7 @@ import androidx.lifecycle.viewModelScope
 import dagger.hilt.android.lifecycle.HiltViewModel
 import de.harheimertc.data.SpielDto
 import de.harheimertc.data.LeagueTableRowDto
+import de.harheimertc.data.SeasonDto
 import de.harheimertc.repositories.Mannschaft
 import de.harheimertc.repositories.MannschaftenRepository
 import de.harheimertc.repositories.SpielplanRepository
@@ -17,6 +18,9 @@ data class MannschaftenUiState(
     val loading: Boolean = true,
     val error: String? = null,
     val teams: List<Mannschaft> = emptyList(),
+    val seasons: List<SeasonDto> = emptyList(),
+    val selectedSeason: String = "",
+    val seasonsLoading: Boolean = false,
 )
 
 @HiltViewModel
@@ -27,17 +31,69 @@ class MannschaftenViewModel @Inject constructor(
     val state: StateFlow<MannschaftenUiState> = _state
 
     init {
-        load()
+        loadSeasonsAndMannschaften()
     }
 
     fun load() {
         viewModelScope.launch {
-            _state.value = MannschaftenUiState(loading = true)
-            repository.fetchMannschaften()
-                .onSuccess { _state.value = MannschaftenUiState(loading = false, teams = it) }
-                .onFailure { _state.value = MannschaftenUiState(loading = false, error = "Mannschaften konnten nicht geladen werden.") }
+            val season = _state.value.selectedSeason.ifBlank { null }
+            _state.value = _state.value.copy(loading = true, error = null)
+            repository.fetchMannschaften(season)
+                .onSuccess { teams -> _state.value = _state.value.copy(loading = false, teams = teams) }
+                .onFailure { _state.value = _state.value.copy(loading = false, error = "Mannschaften konnten nicht geladen werden.") }
         }
     }
+
+    fun selectSeason(season: String) {
+        if (season == _state.value.selectedSeason) return
+        _state.value = _state.value.copy(selectedSeason = season)
+        load()
+    }
+
+    private fun loadSeasonsAndMannschaften() {
+        viewModelScope.launch {
+            _state.value = _state.value.copy(seasonsLoading = true, error = null)
+            repository.fetchSeasons()
+                .onSuccess { response ->
+                    val currentSeason = getCurrentSeasonSlug()
+                    val seasons = response.seasons.ifEmpty { listOf(SeasonDto(slug = response.currentSeason.ifBlank { currentSeason }, label = formatSeasonLabel(response.currentSeason.ifBlank { currentSeason }))) }
+                    val selectedSeason = when {
+                        response.defaultSeason.isNotBlank() -> response.defaultSeason
+                        seasons.any { it.slug == currentSeason } -> currentSeason
+                        seasons.isNotEmpty() -> seasons.first().slug
+                        else -> currentSeason
+                    }
+                    _state.value = _state.value.copy(
+                        seasonsLoading = false,
+                        seasons = seasons,
+                        selectedSeason = selectedSeason,
+                    )
+                    load()
+                }
+                .onFailure {
+                    val currentSeason = getCurrentSeasonSlug()
+                    _state.value = _state.value.copy(
+                        seasonsLoading = false,
+                        seasons = listOf(SeasonDto(slug = currentSeason, label = formatSeasonLabel(currentSeason))),
+                        selectedSeason = currentSeason,
+                    )
+                    load()
+                }
+        }
+    }
+
+    private fun getCurrentSeasonSlug(): String {
+        val now = java.util.Calendar.getInstance()
+        val year = now.get(java.util.Calendar.YEAR)
+        val startYear = if (now.get(java.util.Calendar.MONTH) >= 6) year else year - 1
+        val endYear = startYear + 1
+        return "%02d--%02d".format(startYear % 100, endYear % 100)
+    }
+
+    private fun formatSeasonLabel(seasonSlug: String): String {
+        val match = Regex("^(\\d{2})--(\\d{2})$").matchEntire(seasonSlug.trim()) ?: return seasonSlug
+        return "20${match.groupValues[1]}/${match.groupValues[2]}"
+    }
 }
 
 data class MannschaftDetailUiState(