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/lib/dom.ts

@@ -0,0 +1,9 @@
+export function isTypingTarget(target: EventTarget | null): boolean {
+  if (!(target instanceof HTMLElement)) return false;
+  return (
+    target.tagName === "INPUT" ||
+    target.tagName === "TEXTAREA" ||
+    target.tagName === "SELECT" ||
+    target.isContentEditable
+  );
+}

frontend/src/lib/types.ts

@@ -54,3 +54,13 @@ export interface SystemStatus {
   };
   server_time_ms: number;
 }
+
+export interface FileNeighbor {
+  md5: string;
+  filename: string;
+}
+
+export interface FileNeighbors {
+  previous: FileNeighbor | null;
+  next: FileNeighbor | null;
+}