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

backend/apps/library/views.py

@@ -13,6 +13,8 @@ from . import services
 from .models import MediaItem
 from .serializers import MediaItemSerializer
 
+LIST_ORDERINGS = {"name", "-name", "size", "-size", "created_at", "-created_at"}
+
 
 class MediaItemViewSet(
     mixins.ListModelMixin,
@@ -39,8 +41,9 @@ class MediaItemViewSet(
         if ratings:
             queryset = queryset.filter(rating__in=ratings)
         ordering = self.request.query_params.get("ordering", "").strip()
-        allowed = {"name", "-name", "size", "-size", "created_at", "-created_at"}
-        queryset = queryset.order_by(ordering if ordering in allowed else "-created_at")
+        queryset = queryset.order_by(
+            ordering if ordering in LIST_ORDERINGS else "-created_at"
+        )
         return queryset.distinct()
 
     @action(detail=True, methods=["get"])
@@ -76,6 +79,47 @@ class MediaItemViewSet(
             return services.serve_file(request, thumbnail)
         return services.serve_file(request, path)
 
+    @action(detail=True, methods=["get"])
+    def neighbors(self, request, md5=None):
+        """Previous/next items in the ordered list, for keyboard navigation."""
+        item = self.get_object()
+        ordering = request.query_params.get("ordering", "-created_at").strip()
+        if ordering not in LIST_ORDERINGS:
+            ordering = "-created_at"
+        field = ordering.lstrip("-")
+        descending = ordering.startswith("-")
+        value = {
+            "size": item.size,
+            "created_at": item.created_at,
+            "name": item.name,
+        }[field]
+
+        queryset = self.get_queryset().exclude(pk=item.pk)
+        if descending:
+            previous_query = queryset.filter(**{f"{field}__gt": value}).order_by(field)
+            next_query = queryset.filter(**{f"{field}__lt": value}).order_by(f"-{field}")
+        else:
+            previous_query = queryset.filter(**{f"{field}__lt": value}).order_by(
+                f"-{field}"
+            )
+            next_query = queryset.filter(**{f"{field}__gt": value}).order_by(field)
+
+        def brief(obj):
+            location = obj.locations.first()
+            return {
+                "md5": obj.md5,
+                "filename": Path(location.rel_path).name if location else obj.md5,
+            }
+
+        previous = previous_query.first()
+        next_item = next_query.first()
+        return Response(
+            {
+                "previous": brief(previous) if previous else None,
+                "next": brief(next_item) if next_item else None,
+            }
+        )
+
 
 class UploadView(APIView):
     permission_classes = [IsAuthenticated]