feat: add Group Rectangle — resizable decorative zones on canvas

- New GroupRectNode: rounded rect with NodeResizer (8 handles), no
  connection handles, always behind network nodes via negative zIndex
- GroupRectModal: label, font preset (Inter/Mono/Serif), 3×3 text
  position grid, text/border/background color pickers, z-order 1–9
- Sidebar: "Add Rectangle" button (below Add Node), group rects
  excluded from node count stats
- Save/load: size persisted in custom_colors.width/height, no backend
  schema changes required
- elevateNodesOnSelect=false on ReactFlow so selected rects never
  pop above network nodes
- Tests: 3 new store tests + 9 GroupRectModal tests (66 total, all pass)

frontend/src/components/canvas/CanvasContainer.tsx

@@ -57,6 +57,7 @@ export function CanvasContainer({ onConnect: onConnectProp, onEdgeDoubleClick }:
         snapGrid={[16, 16]}
         fitView
         colorMode="dark"
+        elevateNodesOnSelect={false}
         connectionMode={ConnectionMode.Loose}
         isValidConnection={(connection) => connection.source !== connection.target}
       >