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_files_api.py

@@ -257,6 +257,54 @@ def test_shared_with_me_lists_what_others_shared_and_nothing_else(db, make_user,
     assert [(f["id"], f["shared"]) for f in got] == [(theirs.id, True)]
 
 
+def test_shared_with_me_is_ordered_by_when_it_was_shared_and_can_be_dismissed(
+    db, make_user, as_user
+):
+    """A friend's old file shared today is new to me: "added" is the share's date there. Dismissing
+    one drops only my grant; the owner's file stays."""
+    alice, bob = make_user(), make_user()
+    old = _file(db, bob.id, created_at=NOW - timedelta(days=30))
+    new = _file(db, bob.id, created_at=NOW - timedelta(days=1))
+    db.add(
+        DownloadShare(
+            job_id=new.id,
+            from_user_id=bob.id,
+            to_user_id=alice.id,
+            created_at=NOW - timedelta(hours=2),
+        )
+    )
+    db.add(
+        DownloadShare(
+            job_id=old.id,
+            from_user_id=bob.id,
+            to_user_id=alice.id,
+            created_at=NOW - timedelta(hours=1),
+        )
+    )
+    db.commit()
+    c = as_user(alice)
+    got = c.get("/api/files", params={"scope": "shared"}).json()
+    assert [f["id"] for f in got] == [old.id, new.id]
+    assert got[0]["shared_at"] is not None and got[0]["shared_at"] > got[1]["shared_at"]
+    r = c.delete(f"/api/files/shared/{old.id}")
+    assert r.json() == {"ok": True, "removed": old.id}
+    assert [f["id"] for f in c.get("/api/files", params={"scope": "shared"}).json()] == [new.id]
+    assert db.get(DownloadJob, old.id) is not None
+
+
+def test_each_listed_file_says_where_it_came_from_wherever_it_sits(db, make_user, as_user):
+    """The Source column: a file's own source folder, also after it moved into an own folder."""
+    alice = make_user()
+    yt = _file(db, alice.id)
+    site = _file(db, alice.id, kind="url", ref="https://www.tiktok.com/@x/video/1")
+    moved = _file(db, alice.id)
+    own = _own(db, alice.id)
+    c = as_user(alice)
+    assert c.patch(f"/api/files/{moved.id}", json={"folder_id": own.id}).status_code == 200
+    got = {f["id"]: f["source_key"] for f in c.get("/api/files").json()}
+    assert got == {yt.id: "youtube", site.id: "site:tiktok.com", moved.id: "youtube"}
+
+
 # --- one file ------------------------------------------------------------------------------
 
 
@@ -279,6 +327,34 @@ def test_a_file_is_renamed_kept_and_given_back_to_its_folders_retention(db, make
     )
 
 
+def test_a_files_channel_and_links_are_set_and_only_plausible_links_are_kept(
+    db, make_user, as_user
+):
+    alice = make_user()
+    job = _file(db, alice.id)
+    c = as_user(alice)
+    got = c.patch(
+        f"/api/files/{job.id}",
+        json={
+            "uploader": "  Saját csatorna ",
+            "uploader_url": "mailto:someone@example.com",
+            "extra_links": ["https://a.example/x", "ftp://b.example", "", 7, "http://[::1/x"]
+            + [f"https://c.example/{i}" for i in range(10)],
+        },
+    ).json()
+    assert got["display_uploader"] == "Saját csatorna"
+    assert got["display_uploader_url"] is None
+    assert got["extra_links"][0] == "https://a.example/x" and len(got["extra_links"]) == 8
+    cleared = c.patch(f"/api/files/{job.id}", json={"uploader": "", "extra_links": []}).json()
+    assert (cleared["display_uploader"], cleared["extra_links"]) == (None, [])
+
+
+def test_the_usage_carries_the_quota_and_the_upload_cap(db, make_user, as_user):
+    got = as_user(make_user()).get("/api/files/usage").json()
+    assert got["upload_max_bytes"] == sysconfig.get_int(db, "download_upload_max_bytes")
+    assert "footprint_bytes" in got and "max_bytes" in got
+
+
 def test_a_file_moves_to_an_own_folder_or_home_and_nowhere_else(db, make_user, as_user):
     alice, bob = make_user(), make_user()
     job = _file(db, alice.id)