feat(#527): surface gateway error + background.complete WebSocket events (#550)

Mobile was silently dropping two event types the gateway emits. This adds
parsing + UI surfacing for both, bringing mobile to parity with desktop
(which shows gateway errors as red toasts).

- `error` (backend crash) -> WsEvent.GatewayError, surfaced in the existing
  error banner (mirrors how RpcError is handled).
- `background.complete` (scheduled job finished) -> WsEvent.BackgroundComplete,
  surfaced as a non-blocking snackbar in ChatScreen.

Changes:
- WsEvent.kt: add GatewayError(message?) + BackgroundComplete(data?) sealed subtypes.
- EventParser.kt: handle "error" + "background.complete"; unknowns still fall
  through to Unknown (no behavior change for existing event types).
- ChatWsEventReducer.kt: onGatewayError (sets errorMessage, clears loading) +
  onBackgroundComplete (sets backgroundCompleteMessage); mirrors onRpcError.
- ChatViewModel.kt: backgroundCompleteMessage state field, clearBackgroundComplete(),
  handleWsEvent branches (ViewModel owns the snackbar trigger).
- ChatScreen.kt: pass backgroundCompleteMessage to ChatLifecycleEffects + a
  LaunchedEffect that shows the snackbar and calls clearBackgroundComplete().
- EventParserTest.kt: cover both new events (incl. missing-message case).

Pure additive; sealed class + exhaustive when() preserved. Verified:
compileDebugKotlin + ktlintCheck green; EventParserTest passes.

Closes #527

Co-authored-by: M57 <m57@hermes-control.app>

app/src/main/java/com/m57/hermescontrol/data/ws/EventParser.kt

@@ -111,6 +111,17 @@ object EventParser {
                 WsEvent.StatusUpdate(status, payload)
             }
 
+            "error" -> {
+                val message =
+                    payload?.get("message") as? String
+                        ?: payload?.get("error") as? String
+                WsEvent.GatewayError(message)
+            }
+
+            "background.complete" -> {
+                WsEvent.BackgroundComplete(payload)
+            }
+
             "session.updated" -> {
                 WsEvent.SessionUpdated(payload)
             }

app/src/main/java/com/m57/hermescontrol/data/ws/WsEvent.kt

@@ -125,6 +125,33 @@ sealed class WsEvent {
         val sessionId: String?,
     ) : WsEvent()
 
+    // ── Gateway-level errors ───────────────────────────────────────────
+
+    /**
+     * Backend/unhandled failure surfaced by the gateway.
+     *
+     * Desktop shows these as red toasts (the gateway also writes
+     * `[gateway.error]` lines to the console). Mobile was previously
+     * dropping this event, so a crashing turn just silently stopped with no
+     * explanation. Issue #527 surfaces it in the existing error banner.
+     */
+    data class GatewayError(
+        val message: String?,
+    ) : WsEvent()
+
+    // ── Background job completion ──────────────────────────────────────
+
+    /**
+     * A scheduled/background job finished on the gateway.
+     *
+     * Desktop shows a "Background job finished" toast. Mobile surfaces this as
+     * a non-blocking snackbar. The payload may carry `label`/`name` describing
+     * the job. Issue #527.
+     */
+    data class BackgroundComplete(
+        val data: Map<String, Any?>?,
+    ) : WsEvent()
+
     // ── Fallback ─────────────────────────────────────────────────────────
 
     data class Unknown(

app/src/main/java/com/m57/hermescontrol/ui/chat/ChatScreen.kt

@@ -277,6 +277,7 @@ fun ChatScreen(
         streamingMessage = streamingState.streamingMessage,
         isThinking = streamingState.isThinking,
         errorMessage = state.errorMessage,
+        backgroundCompleteMessage = state.backgroundCompleteMessage,
         isSearchActive = state.isSearchActive,
         currentSearchMatchIndex = state.currentSearchMatchIndex,
         searchMatchIndices = state.searchMatchIndices,
@@ -1390,6 +1391,7 @@ private fun ChatLifecycleEffects(
     streamingMessage: ChatMessage?,
     isThinking: Boolean,
     errorMessage: String?,
+    backgroundCompleteMessage: String?,
     isSearchActive: Boolean,
     currentSearchMatchIndex: Int,
     searchMatchIndices: List<Int>,
@@ -1487,6 +1489,14 @@ private fun ChatLifecycleEffects(
         }
     }
 
+    // Show background-complete as a non-blocking snackbar (issue #527)
+    LaunchedEffect(backgroundCompleteMessage) {
+        backgroundCompleteMessage?.let { message ->
+            snackbarHostState.showSnackbar(message)
+            viewModel.clearBackgroundComplete()
+        }
+    }
+
     // Clarify dialog
     clarifyRequest?.let { clarify ->
         ClarifyDialog(

app/src/main/java/com/m57/hermescontrol/ui/chat/ChatViewModel.kt

@@ -56,6 +56,8 @@ data class ChatUiState(
     /** Standalone streaming message — rendered after the main list. */
     val streamingMessage: ChatMessage? = null,
     val errorMessage: String? = null,
+    // Background job completion toast (issue #527) — non-blocking snackbar
+    val backgroundCompleteMessage: String? = null,
     val clarifyRequest: ClarifyUi? = null,
     // Sudo / secret prompts — surfaced as dialogs (issue #524)
     val sudoPrompt: SudoPromptUi? = null,
@@ -367,6 +369,15 @@ class ChatViewModel(
                 handleSecretRequest(event)
             }
 
+            is WsEvent.GatewayError -> {
+                // Reducer already set errorMessage; no extra VM work needed.
+            }
+
+            is WsEvent.BackgroundComplete -> {
+                // Reducer already set backgroundCompleteMessage; the UI observes
+                // it via a LaunchedEffect and triggers the snackbar.
+            }
+
             else -> { /* reducer handles these */ }
         }
     }
@@ -1033,6 +1044,10 @@ class ChatViewModel(
         _uiState.update { it.copy(errorMessage = null) }
     }
 
+    fun clearBackgroundComplete() {
+        _uiState.update { it.copy(backgroundCompleteMessage = null) }
+    }
+
     // ── Approval flow ───────────────────────────────────────────────────
 
     private fun handleApprovalRequest(event: WsEvent.ApprovalRequest) {

app/src/main/java/com/m57/hermescontrol/ui/chat/ChatWsEventReducer.kt

@@ -71,6 +71,10 @@ object ChatWsEventReducer {
 
             is WsEvent.RpcError -> onRpcError(state, streamingState, event)
 
+            is WsEvent.GatewayError -> onGatewayError(state, streamingState, event)
+
+            is WsEvent.BackgroundComplete -> onBackgroundComplete(state, streamingState, event)
+
             is WsEvent.SessionUpdated -> onSessionUpdated(state, streamingState)
 
             is WsEvent.StatusUpdate -> onStatusUpdate(state, streamingState)
@@ -379,6 +383,44 @@ object ChatWsEventReducer {
             streamingState = streamingState,
         )
 
+    // ── GatewayError ──────────────────────────────────────────────────
+    // Backend/unhandled failure surfaced by the gateway (issue #527).
+    // Mirrors onRpcError: surfaces the message in the existing error banner.
+    private fun onGatewayError(
+        state: ChatUiState,
+        streamingState: StreamingState,
+        event: WsEvent.GatewayError,
+    ): ReducerResult =
+        ReducerResult(
+            state =
+                state.copy(
+                    isLoading = false,
+                    errorMessage = "⚠️ Backend error: ${event.message ?: "Unknown gateway error"}",
+                ),
+            streamingState = streamingState,
+        )
+
+    // ── BackgroundComplete ────────────────────────────────────────────
+    // A scheduled/background job finished (issue #527). The ViewModel turns
+    // this into a non-blocking snackbar. No state mutation beyond passing it
+    // through; the ViewModel owns the snackbar trigger.
+    private fun onBackgroundComplete(
+        state: ChatUiState,
+        streamingState: StreamingState,
+        event: WsEvent.BackgroundComplete,
+    ): ReducerResult {
+        val message =
+            when (val label = event.data?.get("label")) {
+                is String -> label
+                else -> event.data?.get("name") as? String
+            }?.let { "✅ Background job finished: $it" }
+                ?: "✅ Background job finished"
+        return ReducerResult(
+            state = state.copy(backgroundCompleteMessage = message),
+            streamingState = streamingState,
+        )
+    }
+
     // ── SessionUpdated / StatusUpdate / Unknown ───────────────────────
 
     private fun onSessionUpdated(