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/App.tsx

@@ -87,6 +87,7 @@ export default function App() {
       feed: seedLayout("feed", prefs),
       plex: seedLayout("plex", prefs),
       playlists: seedLayout("playlists", prefs),
+      files: seedLayout("files", prefs),
     };
   });
   const [aboutOpen, setAboutOpen] = useState(false);
@@ -119,6 +120,17 @@ export default function App() {
     localKey: LS.playlistsCollapsed,
     parse: Boolean,
   });
+  // The Files rail is the library's folder list — how you move around it, not a filter you set once —
+  // so it starts pinned open (the owner's mockup), and has its own flag.
+  const [filesCollapsed, setFilesCollapsed] = usePref<boolean>("filesCollapsed", false, {
+    localKey: LS.filesCollapsed,
+    parse: Boolean,
+  });
+  const railCollapse = {
+    filter: [filterCollapsed, setFilterCollapsed],
+    playlists: [playlistsCollapsed, setPlaylistsCollapsed],
+    files: [filesCollapsed, setFilesCollapsed],
+  } as const;
   // (The feed filters + page/channel now LIVE in the address bar — FeedFiltersProvider and
   // NavigationProvider read them on load and keep them mirrored — so there's no query to strip here.)
 
@@ -378,18 +390,17 @@ export default function App() {
       />
       {/* The active page's side rail (feed filters / plex filters / playlists list), if it has one.
           A floating collapsible column beside the nav rail; hidden while a channel page overlays the
-          content. feed + plex share the filter-rail collapse flag, playlists has its own. */}
+          content. feed + plex share the filter-rail collapse flag, playlists and files have their own. */}
       {railDef && (
         <Suspense fallback={null}>
           <railDef.Rail
             layout={panelLayouts[railDef.panelId]}
             setLayout={(l) => setPanelLayout(railDef.panelId, l)}
-            collapsed={railDef.collapse === "playlists" ? playlistsCollapsed : filterCollapsed}
-            onToggleCollapse={() =>
-              railDef.collapse === "playlists"
-                ? setPlaylistsCollapsed(!playlistsCollapsed)
-                : setFilterCollapsed(!filterCollapsed)
-            }
+            collapsed={railCollapse[railDef.collapse][0]}
+            onToggleCollapse={() => {
+              const [collapsed, setCollapsed] = railCollapse[railDef.collapse];
+              setCollapsed(!collapsed);
+            }}
           />
         </Suspense>
       )}