feat(files): the Files page — the library in one aligned list, its folders in a rail

Files is the one library: every done file, the ones shared with me, or one
folder's, picked in a side rail (sources, own folders, the storage meter) and
mirrored to ?folder=. One table with every kind of value in its own column;
Copy link, Share and Save in sight, every action on a right click: play,
cut, rename in the row, move to an own folder or back home, keep, details,
re-download, delete.

The Downloads page keeps only the queue until the tray replaces it. The old
/api/downloads library routes are gone: list shared, upload, the details
PATCH, delete, the file and the usage now live under /api/files, and a
listed file says which source it came from. The job's "expired" flag is gone
with the state it described.

frontend/src/lib/queryKeys.ts

@@ -1,4 +1,4 @@
-import type { FeedFilters } from "./api";
+import type { FeedFilters, FileListParams } from "./api";
 
 // Single source of truth for React Query keys. Every key root lives here exactly once, so a typo
 // at one of the ~200 call sites can no longer mint an orphan key that silently never invalidates
@@ -64,11 +64,14 @@ export const qk = {
   // --- sharing ---
   shareRecipients: () => ["share-recipients"] as const,
 
+  // --- files (the library) ---
+  // Every listing's key starts with "files", so one invalidation reaches each folder, search and sort.
+  files: (p?: FileListParams) => (p ? (["files", p] as const) : (["files"] as const)),
+  fileFolders: () => ["file-folders"] as const,
+  storageUsage: () => ["storage-usage"] as const,
   // --- downloads ---
   downloads: () => ["downloads"] as const,
-  downloadsShared: () => ["downloads-shared"] as const,
   downloadIndex: () => ["download-index"] as const,
-  downloadUsage: () => ["download-usage"] as const,
   downloadProfiles: () => ["download-profiles"] as const,
   downloadCredentials: () => ["download-credentials"] as const,
   downloadLinks: (jobId: number) => ["download-links", jobId] as const,