feat: add Notification Settings Modal and Stack Alert Sheet components

- Implemented NotificationSettingsModal for configuring notification agents (Discord, Slack, Webhook) and global settings.
- Added StackAlertSheet for managing stack-specific alert rules with metrics, thresholds, and cooldowns.
- Introduced reusable UI components: DropdownMenu, Popover, Select, Sheet, Slider, and Switch.
- Enhanced Tabs component by removing unnecessary client directive.

backend/src/services/DockerController.ts

@@ -288,6 +288,12 @@ class DockerController {
     });
   }
 
+  public async getContainerStatsStream(containerId: string): Promise<string> {
+    const container = this.docker.getContainer(containerId);
+    const stats = await container.stats({ stream: false });
+    return typeof stats === 'string' ? stats : JSON.stringify(stats);
+  }
+
   /**
    * Exec into a container with full session isolation.
    * All state (exec instance, stream) lives in this closure — no singleton traps.