fix: cleanup WebGL resources before reinitializing viewer on model switch

Address CodeRabbit review feedback:
- Call viewer.cleanup() before initializeStandaloneViewer in watcher to prevent
  WebGL context leaks on 3D-to-3D model switches
- Add { flush: 'post' } to ensure DOM is ready before initialization
- Assert cleanup is called before reinitialization in test
- Unmount wrappers in all tests to prevent cross-test coupling

src/renderer/extensions/linearMode/Preview3d.test.ts

@@ -57,13 +57,15 @@ describe('Preview3d', () => {
   }
 
   it('initializes the viewer on mount', async () => {
-    await mountPreview3d()
+    const wrapper = await mountPreview3d()
 
     expect(initializeStandaloneViewer).toHaveBeenCalledOnce()
     expect(initializeStandaloneViewer).toHaveBeenCalledWith(
       expect.any(HTMLElement),
       'http://localhost/view?filename=model.glb'
     )
+
+    wrapper.unmount()
   })
 
   it('cleans up the viewer on unmount', async () => {
@@ -113,10 +115,13 @@ describe('Preview3d', () => {
     await nextTick()
     await nextTick()
 
+    expect(cleanup).toHaveBeenCalledOnce()
     expect(initializeStandaloneViewer).toHaveBeenCalledOnce()
     expect(initializeStandaloneViewer).toHaveBeenCalledWith(
       expect.any(HTMLElement),
       'http://localhost/view?filename=model-b.glb'
     )
+
+    wrapper.unmount()
   })
 })

src/renderer/extensions/linearMode/Preview3d.vue

@@ -13,11 +13,16 @@ const containerRef = useTemplateRef('containerRef')
 
 const viewer = ref(useLoad3dViewer())
 
-watch([containerRef, () => modelUrl], async () => {
-  if (!containerRef.value || !modelUrl) return
+watch(
+  [containerRef, () => modelUrl],
+  async () => {
+    if (!containerRef.value || !modelUrl) return
 
-  await viewer.value.initializeStandaloneViewer(containerRef.value, modelUrl)
-})
+    viewer.value.cleanup()
+    await viewer.value.initializeStandaloneViewer(containerRef.value, modelUrl)
+  },
+  { flush: 'post' }
+)
 
 onUnmounted(() => {
   viewer.value.cleanup()