feat(ui): customizable sidebar — collapsible, reorderable, toggleable widgets

Each filter group is now a card with a collapse chevron. An Edit mode (pencil)
reveals drag handles (@dnd-kit) to reorder and eye toggles to show/hide widgets,
plus Reset to defaults. Layout (order/collapsed/hidden) persists to localStorage
and the server preferences blob, adopted on login.

frontend/package.json

@@ -8,6 +8,9 @@
     "preview": "vite preview"
   },
   "dependencies": {
+    "@dnd-kit/core": "^6.3.1",
+    "@dnd-kit/sortable": "^10.0.0",
+    "@dnd-kit/utilities": "^3.2.2",
     "@tanstack/react-query": "^5.51.0",
     "clsx": "^2.1.1",
     "lucide-react": "^0.408.0",