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/bodylimit.py

@@ -92,8 +92,7 @@ ROUTE_MAX_BODY_BYTES: dict[tuple[str, str], int] = {
     # So this 413 is a refusal a real user meets on a default install, not a theoretical edge, and
     # it has to be as readable as the route's own: the client localizes `too_large` (`errors.tooLarge`)
     # exactly as it does `upload_too_large`, and the upload dialog refuses an over-cap file before
-    # sending it at all, off the `upload_max_bytes` that `GET /api/downloads/usage` reports.
-    ("POST", "/api/downloads/upload"): settings.download_upload_max_bytes,
+    # sending it at all, off the `upload_max_bytes` that `GET /api/files/usage` reports.
     ("POST", "/api/files/upload"): settings.download_upload_max_bytes,
     ("POST", "/api/debug-sessions/{sid}/screenshots"): debugsession.SCREENSHOT_MAX_BYTES,
 }