feat(web): stale-while-revalidate caches to eliminate tab-switch flicker

Sidebar tabs, Settings, and drill-in detail pages re-fetched on every
visit (loading=true + onMount), flashing an empty skeleton frame on each
navigation. Add an SWR cache layer so revisiting a view renders cached
data instantly while refreshing in the background.

- resourceCache.ts: single-value + keyed (per-id) SWR cache factories
- caches.ts: per-resource cache instances; resetAllCaches() on logout
- eventsSnapshot.ts: warm-seed snapshot for the SSE/paginated events page
- List/sidebar pages read $cache.value via $derived, refresh() on mount;
  mutations refresh the cache
- Settings forms seed once from settingsCache (edit-safe) and refetch
  after save (PUT /api/settings returns {status}, not the Settings object)
- Detail [id] pages warm-seed per id; apps/[id] seeds {workload,containers},
  resets non-seeded panels on warm nav, clears workload on 404, and
  invalidates its cache entry on delete

Deferred (still cold-fetch): triggers/[id] (webhook secret + multi-fetch
body gate), apps/new (create wizard).

web/src/routes/proxies/+page.svelte

@@ -1,6 +1,7 @@
 <script lang="ts">
-	import { listProxyRoutes } from '$lib/api';
+	import { get } from 'svelte/store';
 	import type { ProxyRoute, ProxyRouteSource } from '$lib/types';
+	import { proxyRoutesCache } from '$lib/stores/caches';
 	import { toasts } from '$lib/stores/toast';
 	import { t } from '$lib/i18n';
 	import Skeleton from '$lib/components/Skeleton.svelte';
@@ -10,8 +11,10 @@
 
 	type SourceFilter = 'all' | ProxyRouteSource;
 
-	let routes = $state<ProxyRoute[]>([]);
-	let loading = $state(true);
+	// Cache-backed (stale-while-revalidate) so revisiting this tab renders the
+	// routes immediately instead of flashing the cold skeleton. See caches.ts.
+	const routes = $derived<ProxyRoute[]>($proxyRoutesCache.value);
+	const loading = $derived($proxyRoutesCache.loading);
 	let search = $state('');
 	let sourceFilter = $state<SourceFilter>('all');
 
@@ -66,14 +69,11 @@
 	}
 
 	async function loadRoutes() {
-		loading = true;
-		try {
-			routes = await listProxyRoutes();
-		} catch (err) {
-			toasts.error(err instanceof Error ? err.message : $t('proxies.loadFailed'));
-		} finally {
-			loading = false;
-		}
+		await proxyRoutesCache.refresh();
+		// Cache stores the error rather than throwing; surface it as a toast to
+		// preserve the page's prior failure UX.
+		const { error } = get(proxyRoutesCache);
+		if (error) toasts.error(error || $t('proxies.loadFailed'));
 	}
 
 	$effect(() => {