Shell shortcuts: /, D, [ ], Ctrl+K palette + neighbors endpoint

Backend:
- GET /api/files/{md5}/neighbors/ returns previous/next items in the
  current ordering (name, size, created_at) for keyboard navigation

Frontend:
- / focuses the library search input
- D downloads the file on the detail page
- [ and ] navigate to the previous/next item; hint shown on the page
- Ctrl/Cmd+K opens a command palette (navigation, toggle filters,
  focus search, log in/out); Escape closes it
- Sort order now persists alongside the other library filters so
  prev/next stays consistent

frontend/src/store/ui.ts

@@ -9,6 +9,8 @@ interface UiState {
   setRatings: (ratings: string[]) => void;
   perPage: number;
   setPerPage: (perPage: number) => void;
+  ordering: string;
+  setOrdering: (ordering: string) => void;
 }
 
 export const useUi = create<UiState>()(
@@ -23,6 +25,9 @@ export const useUi = create<UiState>()(
 
       perPage: 48,
       setPerPage: (perPage) => set({ perPage }),
+
+      ordering: "-created_at",
+      setOrdering: (ordering) => set({ ordering }),
     }),
     { name: "j621.ui" },
   ),