feat: filter notifications to warnings/errors/ready only and add clear-all button

Notification filtering:
- lifecycle_hooks.py: only instance.error and instance.health_changed
  with status=running generate notifications. All other lifecycle events
  (created, started, stopped, restarted, deleted) are filtered out.
- health_monitor.py: only error and unhealthy states generate notifications.
  Running/recovered state no longer creates info notifications.
- _derive_title now maps instance.health_changed to "Container ready".

Clear-all button:
- Added dismiss_all() to NotificationService
- Added DELETE /notifications endpoint for bulk dismiss
- Frontend: clearAllNotifications API, clearAll in notification context,
  "Clear all" button in notification drawer alongside "Mark all as read"
- Added CSS for .notification-clear-all with danger hover state
- Updated notification-center tests

Quality gates: pytest (21 passed), vitest (11 passed)

apps/web/src/api/notifications.ts

@@ -30,6 +30,10 @@ export interface MarkAllReadResponse {
 	marked_count: number;
 }
 
+export interface ClearAllResponse {
+	cleared_count: number;
+}
+
 export const getNotifications = async (): Promise<NotificationListResponse> => {
 	const response =
 		await apiClient.get<NotificationListResponse>("/notifications");
@@ -62,3 +66,8 @@ export const markAllNotificationsRead = async (): Promise<number> => {
 export const dismissNotification = async (id: string): Promise<void> => {
 	await apiClient.delete(`/notifications/${id}`);
 };
+
+export const clearAllNotifications = async (): Promise<number> => {
+	const response = await apiClient.delete<ClearAllResponse>("/notifications");
+	return response.data.cleared_count;
+};