feat: container monitoring frontend UI (PR-2)

- Custom ToastContext + ToastProvider + ToastContainer (~170 lines, no deps)
- useEvents() SSE hook with exponential backoff reconnect
- EventProvider context for app-wide SSE stream sharing
- Event-to-toast bridge with severity mapping and deduplication
- Real-time status badge updates replacing 30s polling
- EventSource auth probe (401/429 detection via fetch)
- 14 frontend tests (useEvents + toast-rules)

Quality gates: vitest 14 passed, tsc clean, eslint clean

apps/web/src/api/events.ts

@@ -0,0 +1,22 @@
+const BASE_URL = import.meta.env.VITE_API_BASE_URL ?? "http://localhost:8000";
+
+export function createEventSource(): EventSource {
+	return new EventSource(`${BASE_URL}/events/stream`, {
+		withCredentials: true,
+	});
+}
+
+export async function probeEventStreamStatus(): Promise<number | null> {
+	try {
+		const controller = new AbortController();
+		const timer = setTimeout(() => controller.abort(), 2000);
+		const res = await fetch(`${BASE_URL}/events/stream`, {
+			credentials: "include",
+			signal: controller.signal,
+		});
+		clearTimeout(timer);
+		return res.status;
+	} catch {
+		return null;
+	}
+}