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.

backend/app/logs.py

@@ -690,7 +690,9 @@ _POLLED_ROUTES = frozenset(
         "/api/me/notifications/unread_count",
         "/api/downloads",
         "/api/downloads/index",
-        "/api/downloads/usage",
+        "/api/files",
+        "/api/files/folders",
+        "/api/files/usage",
         "/api/sync/my-status",
         "/api/admin/scheduler",
         "/api/admin/downloads",