fix: auto-refresh remote backup directory when cache is stale (#312)

When entering the cloud backup page, the remote backup directory list only showed cached data and required a manual click of the refresh button to see new backup files. This change adds a TTL-based auto-refresh that fetches fresh data when the cache is older than 5 minutes.

Changes:
- Added refreshedAt tracking per cache key in persisted state
- Added REMOTE_BROWSER_REFRESH_TTL_MS (5 min) constant
- Added useEffect that triggers auto-refresh when destination is selected and cached data is stale
- Stamped refresh timestamps after successful API responses
- Cleaned up timestamps on destination delete and settings save

webapp/src/lib/backup-center.ts

@@ -14,10 +14,12 @@ export interface PersistedRemoteBrowserState {
   pathByDestination: Record<string, string>;
   pageByKey: Record<string, number>;
   selectedDestinationId: string | null;
+  refreshedAt: Record<string, number>;
 }
 
 export const REMOTE_BROWSER_STORAGE_KEY = 'nodewarden.backup.remote-browser.v1';
 export const REMOTE_BROWSER_ITEMS_PER_PAGE = 10;
+export const REMOTE_BROWSER_REFRESH_TTL_MS = 5 * 60 * 1000; // 5 minutes
 
 export const COMMON_TIME_ZONES = [
   'UTC',
@@ -148,6 +150,7 @@ export function loadPersistedRemoteBrowserState(userId?: string | null): Persist
         pathByDestination: {},
         pageByKey: {},
         selectedDestinationId: null,
+        refreshedAt: {},
       };
     }
     const parsed = JSON.parse(raw) as Partial<PersistedRemoteBrowserState>;
@@ -156,6 +159,7 @@ export function loadPersistedRemoteBrowserState(userId?: string | null): Persist
       pathByDestination: parsed.pathByDestination && typeof parsed.pathByDestination === 'object' ? parsed.pathByDestination : {},
       pageByKey: parsed.pageByKey && typeof parsed.pageByKey === 'object' ? parsed.pageByKey : {},
       selectedDestinationId: typeof parsed.selectedDestinationId === 'string' ? parsed.selectedDestinationId : null,
+      refreshedAt: parsed.refreshedAt && typeof parsed.refreshedAt === 'object' ? parsed.refreshedAt as Record<string, number> : {},
     };
   } catch {
     return {
@@ -163,6 +167,7 @@ export function loadPersistedRemoteBrowserState(userId?: string | null): Persist
       pathByDestination: {},
       pageByKey: {},
       selectedDestinationId: null,
+      refreshedAt: {},
     };
   }
 }