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/settings/maintenance/+page.svelte

@@ -6,7 +6,11 @@
   never within casual miss-click distance of general form fields.
 -->
 <script lang="ts">
-	import { getSettings, updateSettings, pruneImages, pruneBuildCache } from '$lib/api';
+	import { onMount } from 'svelte';
+	import { get } from 'svelte/store';
+	import { updateSettings, pruneImages, pruneBuildCache } from '$lib/api';
+	import type { Settings } from '$lib/types';
+	import { settingsCache } from '$lib/stores/caches';
 	import FormField from '$lib/components/FormField.svelte';
 	import ConfirmDialog from '$lib/components/ConfirmDialog.svelte';
 	import Skeleton from '$lib/components/Skeleton.svelte';
@@ -26,19 +30,28 @@
 	let statsIntervalSeconds = $state('15');
 	let statsRetentionHours = $state('2');
 
+	// Seed form fields once from the shared settings cache (warm → no skeleton
+	// on revisit); never re-applied during background refresh, so unsaved edits
+	// are safe. See caches.ts / settings landing page for the pattern.
+	function seed(s: Settings) {
+		staleThresholdDays = String(s.stale_threshold_days ?? 7);
+		imagePruneThresholdMb = String(s.image_prune_threshold_mb ?? 1024);
+		statsIntervalSeconds = String(s.stats_interval_seconds ?? 15);
+		statsRetentionHours = String(s.stats_retention_hours ?? 2);
+	}
+
 	async function load() {
-		loading = true;
-		try {
-			const s = await getSettings();
-			staleThresholdDays = String(s.stale_threshold_days ?? 7);
-			imagePruneThresholdMb = String(s.image_prune_threshold_mb ?? 1024);
-			statsIntervalSeconds = String(s.stats_interval_seconds ?? 15);
-			statsRetentionHours = String(s.stats_retention_hours ?? 2);
-		} catch (err) {
-			toasts.error(err instanceof Error ? err.message : $t('settingsGeneral.loadFailed'));
-		} finally {
+		const cached = get(settingsCache).value;
+		if (cached) {
+			seed(cached);
 			loading = false;
 		}
+		await settingsCache.refresh();
+		const fresh = get(settingsCache).value;
+		if (fresh && loading) seed(fresh);
+		loading = false;
+		const { error } = get(settingsCache);
+		if (error && !cached) toasts.error(error || $t('settingsGeneral.loadFailed'));
 	}
 
 	async function handleSave() {
@@ -59,6 +72,8 @@
 				stats_interval_seconds: interval,
 				stats_retention_hours: retention
 			});
+			// PUT returns {status:"updated"}, not Settings — refetch the cache.
+			await settingsCache.refresh();
 			toasts.success($t('settingsGeneral.saved'));
 		} catch (err) {
 			toasts.error(err instanceof Error ? err.message : $t('settingsGeneral.saveFailed'));
@@ -91,7 +106,7 @@
 		}
 	}
 
-	$effect(() => { load(); });
+	onMount(load);
 </script>
 
 <svelte:head>