perf(library): persistent SPA shell, in-memory folder tree caching, and zero-flicker LRU thumbnail caching

docker/frontend/src/engine/library.ts

@@ -146,7 +146,7 @@ export function canBrowseFolders(): boolean {
   return typeof window !== 'undefined' && 'showDirectoryPicker' in window;
 }
 
-const THUMB_CACHE_MAX = 500;
+const THUMB_CACHE_MAX = 5000;
 const thumbCache = new Map<string, Blob>();
 
 export function cacheThumb(id: string, blob: Blob): void {
@@ -278,14 +278,20 @@ function ask<T>(store: string, mode: IDBTransactionMode, run: (s: IDBObjectStore
   );
 }
 
-// --- folders ---------------------------------------------------------------
+let cachedFoldersList: LibraryFolder[] | null = null;
+
+export function cachedFolders(): LibraryFolder[] {
+  return cachedFoldersList ?? [];
+}
 
 export async function listFolders(): Promise<LibraryFolder[]> {
   try {
     const rows = await ask<LibraryFolder[]>(FOLDERS, 'readonly', (s) => s.getAll());
-    return rows.sort((a, b) => a.name.localeCompare(b.name));
+    const sorted = rows.sort((a, b) => a.name.localeCompare(b.name));
+    cachedFoldersList = sorted;
+    return sorted;
   } catch {
-    return [];
+    return cachedFoldersList ?? [];
   }
 }