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/components/event-toast-bridge.tsx

@@ -0,0 +1,19 @@
+import { useEffect, useRef } from "react";
+import { useEventContext } from "../state/events";
+import { handleEventToast } from "./toast-rules";
+
+export function EventToastBridge(): JSX.Element | null {
+	const { events } = useEventContext();
+	const processedRef = useRef<Set<string>>(new Set());
+
+	useEffect(() => {
+		for (const event of events) {
+			const key = `${event.correlation_id}:${event.timestamp}`;
+			if (processedRef.current.has(key)) continue;
+			processedRef.current.add(key);
+			handleEventToast(event);
+		}
+	}, [events]);
+
+	return null;
+}