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/+layout.svelte

@@ -13,6 +13,7 @@
 	import { logout as apiLogout } from '$lib/api';
 	import { t } from '$lib/i18n';
 	import { navCounts, startNavCountsPolling, stopNavCountsPolling, refreshNavCounts } from '$lib/stores/navCounts';
+	import { resetAllCaches } from '$lib/stores/caches';
 	import { health, startHealthPolling, stopHealthPolling, refreshHealth } from '$lib/stores/health';
 	import { effectiveTimezone, formatOffsetLabel } from '$lib/stores/timezone';
 	import { fmt } from '$lib/format/datetime';
@@ -357,6 +358,7 @@
 					onclick={async () => {
 						try { await apiLogout(); } catch { /* best effort */ }
 						clearAuth();
+						resetAllCaches();
 						goto('/login');
 					}}
 					class="rounded-md p-1.5 text-[var(--text-tertiary)] hover:text-[var(--color-danger)] transition-colors"