fix(websocket): Reconnect-Strategie robuster machen

WebSocketClientImpl:
- Jitter (±25%) zum exponentiellen Backoff hinzugefügt, um
  Thundering-Herd-Effekt bei Server-Neustart zu vermeiden
- Retry-Zähler mit MAX_RETRIES=5: nach 5 konsekutiven Fehlschlägen
  wird ConnectionFailed-Event emittiert (nach ~62s)
- Client versucht weiterhin im MAX_BACKOFF-Intervall (60s) zu
  reconnecten, gibt nicht vollständig auf
- Zähler wird bei erfolgreicher Verbindung zurückgesetzt

WebSocketEvent: neues ConnectionFailed(message) Event hinzugefügt

SettingsViewModel: ConnectionFailed -> SyncStatus.Error,
Connected -> SyncStatus.Idle (Fehler wird beim Reconnect gelöscht)

Closes #73

app/src/main/java/de/krisenvorrat/app/data/sync/WebSocketClient.kt

@@ -14,5 +14,6 @@ internal sealed interface WebSocketEvent {
     data object FullSyncRequired : WebSocketEvent
     data object Connected : WebSocketEvent
     data object Disconnected : WebSocketEvent
+    data class ConnectionFailed(val message: String) : WebSocketEvent
     data class NewMessage(val message: MessageDto) : WebSocketEvent
 }

app/src/main/java/de/krisenvorrat/app/data/sync/WebSocketClientImpl.kt

@@ -22,6 +22,7 @@ import kotlinx.serialization.Serializable
 import kotlinx.serialization.json.Json
 import javax.inject.Inject
 import javax.inject.Singleton
+import kotlin.random.Random
 
 @Singleton
 internal class WebSocketClientImpl @Inject constructor() : WebSocketClient {
@@ -40,6 +41,8 @@ internal class WebSocketClientImpl @Inject constructor() : WebSocketClient {
         connectionJob?.cancel()
         connectionJob = scope.launch {
             var backoffMs = INITIAL_BACKOFF_MS
+            var consecutiveFailures = 0
+            var connectionFailedEmitted = false
             while (isActive) {
                 try {
                     val wsUrl = serverUrl.trimEnd('/')
@@ -47,6 +50,8 @@ internal class WebSocketClientImpl @Inject constructor() : WebSocketClient {
                         .replace("http://", "ws://")
                     wsHttpClient.webSocket("$wsUrl/ws/sync?token=$accessToken") {
                         backoffMs = INITIAL_BACKOFF_MS
+                        consecutiveFailures = 0
+                        connectionFailedEmitted = false
                         _events.emit(WebSocketEvent.Connected)
                         for (frame in incoming) {
                             if (frame is Frame.Text) {
@@ -57,11 +62,20 @@ internal class WebSocketClientImpl @Inject constructor() : WebSocketClient {
                 } catch (e: CancellationException) {
                     break
                 } catch (_: Exception) {
-                    // connection failed – retry after backoff
+                    consecutiveFailures++
+                    if (consecutiveFailures >= MAX_RETRIES && !connectionFailedEmitted) {
+                        _events.emit(
+                            WebSocketEvent.ConnectionFailed(
+                                "Verbindung nach $MAX_RETRIES Versuchen fehlgeschlagen"
+                            )
+                        )
+                        connectionFailedEmitted = true
+                    }
                 }
                 if (!isActive) break
                 _events.emit(WebSocketEvent.Disconnected)
-                delay(backoffMs)
+                val jitter = backoffMs * JITTER_FACTOR * (Random.nextDouble() * 2 - 1)
+                delay(backoffMs + jitter.toLong())
                 backoffMs = minOf(backoffMs * 2, MAX_BACKOFF_MS)
             }
         }
@@ -100,6 +114,8 @@ internal class WebSocketClientImpl @Inject constructor() : WebSocketClient {
     private companion object {
         const val INITIAL_BACKOFF_MS = 2_000L
         const val MAX_BACKOFF_MS = 60_000L
+        const val MAX_RETRIES = 5
+        const val JITTER_FACTOR = 0.25
         val json = Json { ignoreUnknownKeys = true }
     }
 }

app/src/main/java/de/krisenvorrat/app/ui/settings/SettingsViewModel.kt

@@ -53,6 +53,12 @@ internal class SettingsViewModel @Inject constructor(
                 when (event) {
                     is WebSocketEvent.FullSyncRequired -> pullSync()
                     is WebSocketEvent.InventoryUpdated -> pullSync()
+                    is WebSocketEvent.ConnectionFailed -> {
+                        _uiState.update { it.copy(syncStatus = SyncStatus.Error(event.message)) }
+                    }
+                    is WebSocketEvent.Connected -> {
+                        _uiState.update { it.copy(syncStatus = SyncStatus.Idle) }
+                    }
                     else -> {}
                 }
             }