feat: Entity 선택기능, Property Inspector 패널, Layer 관리 패널, 드래그/접기/크기조절 UI 구현

src/viewer2d/Viewer2D.d.ts

@@ -11,6 +11,9 @@ export class Viewer2D {
   /** Resync renderer/camera to the container size (call after un-hiding). */
   resize(): void;
   onSelect(cb: (entity: unknown) => void): void;
+  getSelectedEntity(): unknown;
+  zoomToSelection(): void;
+  setAccent(hex: string): void;
   setTheme(dark: boolean): void;
   setGrid(visible: boolean): void;
   getLayerInfo(): { name: string; colorHex: string; count: number; visible: boolean }[];