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_phone_safe_contract.py

@@ -141,9 +141,7 @@ def test_what_a_link_serves_passes_the_rule(
     subprocess.run(["ffmpeg", "-y", "-loglevel", "error", *args, str(src)], check=True)
 
     owner = as_user(make_user())
-    r = owner.post(
-        "/api/downloads/upload", content=src.read_bytes(), headers=upload_headers(filename)
-    )
+    r = owner.post("/api/files/upload", content=src.read_bytes(), headers=upload_headers(filename))
     assert r.status_code == 200, r.text
     job_id = r.json()["id"]
     assert r.json()["status"] == "queued"  # every one of these fails the rule as it came