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(