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/tests/test_spa_entries.py

@@ -34,7 +34,7 @@ def link_token(as_user, make_user, tmp_path, monkeypatch, upload_headers):
     monkeypatch.setattr(probemod, "probe_json", lambda _p: data)
     owner = as_user(make_user())
     job = owner.post(
-        "/api/downloads/upload",
+        "/api/files/upload",
         content=b"fake-video",
         headers=upload_headers("clip.mp4", display_name="Holiday clip"),
     )