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/components/AppShell.tsx

@@ -1,8 +1,9 @@
 import { useQuery } from "@tanstack/react-query";
 import { LogOut, Menu } from "lucide-react";
-import { useEffect } from "react";
+import { useEffect, useState } from "react";
 import { Link, NavLink, Outlet, useLocation } from "react-router-dom";
 
+import { CommandPalette } from "@/components/CommandPalette";
 import { StatusFooter } from "@/components/StatusFooter";
 import { StatusPill } from "@/components/StatusPill";
 import { api } from "@/lib/api";
@@ -21,6 +22,7 @@ export function AppShell() {
   const logout = useAuth((state) => state.logout);
   const toggleSidebar = useUi((state) => state.toggleSidebar);
   const location = useLocation();
+  const [paletteOpen, setPaletteOpen] = useState(false);
 
   const statusQuery = useQuery({
     queryKey: ["status"],
@@ -32,9 +34,19 @@ export function AppShell() {
 
   useEffect(() => {
     function handleKeyDown(event: KeyboardEvent) {
-      if ((event.ctrlKey || event.metaKey) && event.key.toLowerCase() === "b") {
+      const modifier = event.ctrlKey || event.metaKey;
+      if (modifier && event.key.toLowerCase() === "b") {
         event.preventDefault();
         toggleSidebar();
+        return;
+      }
+      if (modifier && event.key.toLowerCase() === "k") {
+        event.preventDefault();
+        setPaletteOpen((open) => !open);
+        return;
+      }
+      if (event.key === "Escape") {
+        setPaletteOpen(false);
       }
     }
     window.addEventListener("keydown", handleKeyDown);
@@ -122,6 +134,7 @@ export function AppShell() {
       </main>
 
       <StatusFooter status={status} />
+      <CommandPalette open={paletteOpen} onClose={() => setPaletteOpen(false)} />
     </div>
   );
 }