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/state/events.tsx

@@ -0,0 +1,30 @@
+import React, { createContext, useContext, useMemo } from "react";
+import { useEvents } from "../hooks/use-events";
+import type { InstanceEventPayload } from "../types/events";
+
+interface EventContextValue {
+	events: InstanceEventPayload[];
+	connected: boolean;
+	reconnectCount: number;
+}
+
+const EventContext = createContext<EventContextValue>({
+	events: [],
+	connected: false,
+	reconnectCount: 0,
+});
+
+export function EventProvider({ children }: { children: React.ReactNode }) {
+	const { events, connected, reconnectCount } = useEvents();
+	const value = useMemo(
+		() => ({ events, connected, reconnectCount }),
+		[events, connected, reconnectCount],
+	);
+	return (
+		<EventContext.Provider value={value}>{children}</EventContext.Provider>
+	);
+}
+
+export function useEventContext() {
+	return useContext(EventContext);
+}