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/styles.css

@@ -3381,6 +3381,17 @@ a.nav-item,
 	color: var(--danger, #dc2626);
 }
 
+.status-badge.starting,
+.status-badge.probing {
+	background: var(--info-light, #dbeafe);
+	color: var(--info, #2563eb);
+}
+
+.status-badge.unhealthy {
+	background: var(--warning-light, #fef3c7);
+	color: var(--warning, #d97706);
+}
+
 /* ============================================
    Mobile Terminal Styles
    ============================================ */
@@ -4452,3 +4463,26 @@ a:active,
 	gap: 0.5rem;
 	margin-top: 0.25rem;
 }
+
+/* Toast animations */
+@keyframes toastSlideIn {
+	from {
+		opacity: 0;
+		transform: translateX(100%);
+	}
+	to {
+		opacity: 1;
+		transform: translateX(0);
+	}
+}
+
+@keyframes toastFadeOut {
+	from {
+		opacity: 1;
+		transform: translateX(0);
+	}
+	to {
+		opacity: 0;
+		transform: translateX(100%);
+	}
+}