feat: notification center toast coordination (PR-4)

- EventToastBridge checks notification_toast_level and notification_mute_categories
- toast-rules.ts: event-to-category/severity mapping functions
- Settings page: notification preferences section (toast level dropdown, mute checkboxes)
- Settings API types extended with notification preference fields
- 17 frontend tests (toast-rules + bridge)
- Preference hierarchy: mute categories → toast level → show/hide

Quality gates: vitest 17 passed, tsc clean, eslint clean

apps/web/src/components/toast-rules.ts

@@ -19,6 +19,32 @@ function shouldShowToast(instanceId: string, eventType: string): boolean {
 	return true;
 }
 
+export function mapEventToCategory(event: InstanceEventPayload): string {
+	if (event.event.startsWith("instance.")) return "instance";
+	if (event.event.startsWith("health.")) return "health";
+	return "system";
+}
+
+export function mapEventToSeverity(
+	event: InstanceEventPayload,
+): "info" | "warning" | "error" | "success" {
+	switch (event.event) {
+		case "instance.error":
+		case "health.error":
+			return "error";
+		case "instance.health_changed":
+			return event.status === "unhealthy" ? "warning" : "success";
+		case "instance.created":
+		case "instance.started":
+		case "instance.stopped":
+		case "instance.restarted":
+		case "instance.deleted":
+			return "info";
+		default:
+			return "info";
+	}
+}
+
 export function handleEventToast(event: InstanceEventPayload): void {
 	const { event: eventType, instance_id, status, message, metadata } = event;