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.

frontend/src/components/Header.tsx

@@ -3,6 +3,7 @@ import { useTranslation } from "react-i18next";
 import { YouTubeIcon } from "../lib/brandIcons";
 import type { Me } from "../lib/api";
 import SearchBar from "./SearchBar";
+import { useFiles, useFilesActions } from "./filesContext";
 import PlexSearchBox from "./PlexSearchBox";
 import PlexScopeToggle from "./PlexScopeToggle";
 import PlexViewToggle from "./PlexViewToggle";
@@ -37,6 +38,8 @@ const Header = forwardRef<
   const { search: playlistsSearch } = usePlaylists();
   const { setSearch: setPlaylistsSearch } = usePlaylistsActions();
   const { search: channelsSearch } = useChannels();
+  const { search: filesSearch } = useFiles();
+  const { setSearch: setFilesSearch } = useFilesActions();
   const { setSearch: setChannelsSearch, goToFullHistory: onGoToFullHistory } = useChannelsActions();
   // Drop input focus on Go/Enter for the live-filter modules (feed's Go escalates to YouTube).
   const blur = () => (document.activeElement as HTMLElement | null)?.blur?.();
@@ -77,6 +80,15 @@ const Header = forwardRef<
         onGo={blur}
       />
     );
+  } else if (page === "files") {
+    search = (
+      <SearchBar
+        value={filesSearch}
+        placeholder={t("files.searchPlaceholder")}
+        onChange={setFilesSearch}
+        onGo={blur}
+      />
+    );
   } else if (page === "playlists") {
     search = (
       <SearchBar