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/CommandPalette.tsx

@@ -0,0 +1,180 @@
+import {
+  FolderOpen,
+  LogIn,
+  LogOut,
+  Menu,
+  Search,
+  Upload,
+  type LucideIcon,
+} from "lucide-react";
+import { useMemo, useState } from "react";
+import { useNavigate } from "react-router-dom";
+
+import { cn } from "@/lib/cn";
+import { useAuth } from "@/store/auth";
+import { useUi } from "@/store/ui";
+
+interface Command {
+  id: string;
+  label: string;
+  hint?: string;
+  icon: LucideIcon;
+  run: () => void;
+}
+
+export function CommandPalette({
+  open,
+  onClose,
+}: {
+  open: boolean;
+  onClose: () => void;
+}) {
+  if (!open) return null;
+  return <CommandPaletteDialog onClose={onClose} />;
+}
+
+function CommandPaletteDialog({ onClose }: { onClose: () => void }) {
+  const navigate = useNavigate();
+  const user = useAuth((state) => state.user);
+  const logout = useAuth((state) => state.logout);
+  const toggleSidebar = useUi((state) => state.toggleSidebar);
+  const [query, setQuery] = useState("");
+  const [activeIndex, setActiveIndex] = useState(0);
+
+  const commands = useMemo<Command[]>(() => {
+    const list: Command[] = [
+      {
+        id: "library",
+        label: "Go to Library",
+        icon: FolderOpen,
+        run: () => navigate("/"),
+      },
+      {
+        id: "upload",
+        label: "Go to Upload",
+        icon: Upload,
+        run: () => navigate("/upload"),
+      },
+      {
+        id: "filters",
+        label: "Toggle filters",
+        hint: "Ctrl+B",
+        icon: Menu,
+        run: toggleSidebar,
+      },
+      {
+        id: "search",
+        label: "Focus library search",
+        hint: "/",
+        icon: Search,
+        run: () => {
+          navigate("/");
+          window.requestAnimationFrame(() => {
+            document.getElementById("library-search")?.focus();
+          });
+        },
+      },
+    ];
+    if (user) {
+      list.push({
+        id: "logout",
+        label: `Log out (${user.username})`,
+        icon: LogOut,
+        run: () => void logout(),
+      });
+    } else {
+      list.push({
+        id: "login",
+        label: "Log in",
+        icon: LogIn,
+        run: () => navigate("/login"),
+      });
+    }
+    return list;
+  }, [navigate, toggleSidebar, user, logout]);
+
+  const filtered = useMemo(() => {
+    const needle = query.trim().toLowerCase();
+    return needle
+      ? commands.filter((command) =>
+          command.label.toLowerCase().includes(needle),
+        )
+      : commands;
+  }, [commands, query]);
+
+  function runCommand(command: Command | undefined) {
+    if (!command) return;
+    onClose();
+    command.run();
+  }
+
+  return (
+    <div
+      className="fixed inset-0 z-[60] flex items-start justify-center bg-ctp-crust/70 px-4 pt-[18vh] backdrop-blur-sm"
+      onClick={onClose}
+    >
+      <div
+        className="w-full max-w-lg overflow-hidden rounded-lg border border-ctp-surface1 bg-ctp-surface0 shadow-[0_4px_12px_rgba(17,17,27,0.6)]"
+        onClick={(event) => event.stopPropagation()}
+      >
+        <input
+          autoFocus
+          value={query}
+          onChange={(event) => {
+            setQuery(event.target.value);
+            setActiveIndex(0);
+          }}
+          onKeyDown={(event) => {
+            if (event.key === "ArrowDown") {
+              event.preventDefault();
+              setActiveIndex((index) =>
+                Math.min(index + 1, filtered.length - 1),
+              );
+            } else if (event.key === "ArrowUp") {
+              event.preventDefault();
+              setActiveIndex((index) => Math.max(index - 1, 0));
+            } else if (event.key === "Enter") {
+              event.preventDefault();
+              runCommand(filtered[activeIndex]);
+            } else if (event.key === "Escape") {
+              onClose();
+            }
+          }}
+          placeholder="Type a command…"
+          className="w-full border-b border-ctp-surface1 bg-transparent px-4 py-3 font-mono text-sm text-ctp-text placeholder:text-ctp-overlay0 focus:outline-none"
+        />
+        <ul className="max-h-72 overflow-y-auto p-1.5">
+          {filtered.length === 0 ? (
+            <li className="px-3 py-6 text-center text-sm text-ctp-overlay0">
+              No matching commands
+            </li>
+          ) : (
+            filtered.map((command, index) => (
+              <li key={command.id}>
+                <button
+                  type="button"
+                  onMouseEnter={() => setActiveIndex(index)}
+                  onClick={() => runCommand(command)}
+                  className={cn(
+                    "flex w-full items-center gap-3 rounded-md px-3 py-2 text-left text-sm transition",
+                    index === activeIndex
+                      ? "bg-ctp-surface1 text-ctp-text"
+                      : "text-ctp-subtext0",
+                  )}
+                >
+                  <command.icon className="h-4 w-4 shrink-0" />
+                  <span className="flex-1 truncate">{command.label}</span>
+                  {command.hint ? (
+                    <span className="font-mono text-[11px] text-ctp-overlay0">
+                      {command.hint}
+                    </span>
+                  ) : null}
+                </button>
+              </li>
+            ))
+          )}
+        </ul>
+      </div>
+    </div>
+  );
+}