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/hooks/use-events.ts

@@ -0,0 +1,108 @@
+import { useEffect, useRef, useState, useCallback } from "react";
+import { createEventSource, probeEventStreamStatus } from "../api/events";
+import type { InstanceEventPayload } from "../types/events";
+
+export interface UseEventsReturn {
+	events: InstanceEventPayload[];
+	connected: boolean;
+	reconnectCount: number;
+	error: Error | null;
+}
+
+const MAX_DELAY = 30000;
+const BASE_DELAY = 1000;
+
+export function useEvents(): UseEventsReturn {
+	const [events, setEvents] = useState<InstanceEventPayload[]>([]);
+	const [connected, setConnected] = useState(false);
+	const [reconnectCount, setReconnectCount] = useState(0);
+	const [error, setError] = useState<Error | null>(null);
+	const reconnectAttemptsRef = useRef(0);
+	const esRef = useRef<EventSource | null>(null);
+	const timeoutRef = useRef<ReturnType<typeof setTimeout> | null>(null);
+	const isMountedRef = useRef(true);
+
+	const connect = useCallback(() => {
+		if (timeoutRef.current) {
+			clearTimeout(timeoutRef.current);
+			timeoutRef.current = null;
+		}
+
+		const es = createEventSource();
+		esRef.current = es;
+
+		es.onopen = () => {
+			if (!isMountedRef.current) return;
+			setConnected(true);
+			setError(null);
+			reconnectAttemptsRef.current = 0;
+			setReconnectCount(0);
+		};
+
+		es.onmessage = (e) => {
+			if (!isMountedRef.current) return;
+			try {
+				const payload: InstanceEventPayload = JSON.parse(e.data);
+				setEvents((prev) => [...prev, payload]);
+			} catch {
+				// ignore malformed events
+			}
+		};
+
+		es.onerror = () => {
+			if (!isMountedRef.current) return;
+			setConnected(false);
+			es.close();
+			esRef.current = null;
+
+			const attempts = reconnectAttemptsRef.current;
+			const delay =
+				Math.min(MAX_DELAY, BASE_DELAY * Math.pow(2, attempts)) *
+				(0.8 + Math.random() * 0.4);
+			reconnectAttemptsRef.current = attempts + 1;
+			setReconnectCount(reconnectAttemptsRef.current);
+
+			timeoutRef.current = setTimeout(async () => {
+				if (!isMountedRef.current) return;
+
+				const status = await probeEventStreamStatus();
+				if (status === 401) {
+					const baseUrl =
+						import.meta.env.VITE_API_BASE_URL ?? "http://localhost:8000";
+					window.location.assign(`${baseUrl}/auth/login`);
+					return;
+				}
+
+				if (status === 429) {
+					const penaltyDelay = delay + 5000;
+					timeoutRef.current = setTimeout(() => {
+						if (isMountedRef.current) {
+							connect();
+						}
+					}, penaltyDelay);
+					return;
+				}
+
+				connect();
+			}, delay);
+		};
+	}, []);
+
+	useEffect(() => {
+		isMountedRef.current = true;
+		connect();
+		return () => {
+			isMountedRef.current = false;
+			if (timeoutRef.current) {
+				clearTimeout(timeoutRef.current);
+				timeoutRef.current = null;
+			}
+			if (esRef.current) {
+				esRef.current.close();
+				esRef.current = null;
+			}
+		};
+	}, [connect]);
+
+	return { events, connected, reconnectCount, error };
+}