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_body_limit.py

@@ -367,7 +367,7 @@ def test_every_body_reading_site_is_known_and_fits_under_its_ceiling():
     assert ("POST", "/api/debug-sessions/{sid}/screenshots") in bodylimit.ROUTE_MAX_BODY_BYTES
     # The upload site's limit is the sysconfig knob, which is bounded BY the edge ceiling rather
     # than being a constant to compare against — that pairing is its own test below.
-    assert ("POST", "/api/downloads/upload") in bodylimit.ROUTE_MAX_BODY_BYTES
+    assert ("POST", "/api/files/upload") in bodylimit.ROUTE_MAX_BODY_BYTES
 
 
 def test_release_read_connection_ends_the_transaction_it_was_called_for(db):
@@ -487,7 +487,7 @@ def test_the_upload_ceiling_and_the_admin_knob_cannot_drift_apart():
     `errors.tooLarge` exists and why the dialog checks the file against `upload_max_bytes` first.
     """
     spec = next(s for s in sysconfig.SPECS if s.key == "download_upload_max_bytes")
-    edge = bodylimit.ROUTE_MAX_BODY_BYTES[("POST", "/api/downloads/upload")]
+    edge = bodylimit.ROUTE_MAX_BODY_BYTES[("POST", "/api/files/upload")]
     assert spec.max == edge