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/models.py

@@ -39,7 +39,7 @@ def display_text(raw: str | None) -> str | None:
     blank -> None.
 
     One definition because there are SIX writers and they had drifted: `POST /api/downloads`,
-    `POST /api/downloads/edit`, `POST /api/downloads/upload`, the metadata PATCH's two fields, and
+    `POST /api/downloads/edit`, `POST /api/files/upload`, the library PATCH's two fields, and
     the Google sign-in callback. Only the PATCH truncated. Truncating is the right answer rather
     than refusing: the name is a label on work the user has already done — a file already
     transferred, an account already authenticated — and losing its tail is not worth failing the