perf: virtualize FormDropdownMenu and fix VirtualGrid jitter

- Integrate VirtualGrid into FormDropdownMenu for virtualized rendering
- Fix jitter: overflow-anchor:none, scrollbar-gutter:stable, cols=maxColumns when finite
- Remove transition-[width] from grid items, replace transition-all with explicit properties
- Replace LazyImage with native img (redundant with virtualization)
- Change max-h-[640px] to fixed h-[640px] for proper virtualization
- Add unit tests for VirtualGrid and FormDropdownMenu
- Add E2E test for image dropdown virtualization

Amp-Thread-ID: https://ampcode.com/threads/T-019c5a71-66c8-76e9-95ed-671a1b4538da

browser_tests/tests/vueNodes/widgets/imageDropdownVirtualization.spec.ts

@@ -0,0 +1,49 @@
+import {
+  comfyExpect as expect,
+  comfyPageFixture as test
+} from '../../../fixtures/ComfyPage'
+
+test.describe('Image Dropdown Virtualization', { tag: '@widget' }, () => {
+  test.beforeEach(async ({ comfyPage }) => {
+    await comfyPage.setSetting('Comfy.VueNodes.Enabled', true)
+    await comfyPage.vueNodes.waitForNodes()
+  })
+
+  test('should virtualize items when dropdown has many entries', async ({
+    comfyPage
+  }) => {
+    await comfyPage.loadWorkflow('widgets/load_image_widget')
+    await comfyPage.vueNodes.waitForNodes()
+
+    const totalItems = await comfyPage.page.evaluate(() => {
+      const node = window['graph']._nodes_by_id['10']
+      const widget = node.widgets.find(
+        (w: { name: string }) => w.name === 'image'
+      )
+      const count = 60
+      const values = Array.from(
+        { length: count },
+        (_, i) => `test_image_${i}.png`
+      )
+      widget.options.values = values
+      widget.value = values[0]
+      return count
+    })
+
+    const loadImageNode = comfyPage.vueNodes.getNodeByTitle('Load Image')
+    const dropdownButton = loadImageNode.locator(
+      'button:has(span:has-text("test_image_0.png"))'
+    )
+    await dropdownButton.waitFor({ state: 'visible' })
+    await dropdownButton.click()
+
+    const virtualGridItems = comfyPage.page.locator(
+      '[data-virtual-grid-item]'
+    )
+    await expect(virtualGridItems.first()).toBeVisible()
+
+    const renderedCount = await virtualGridItems.count()
+    expect(renderedCount).toBeLessThan(totalItems)
+    expect(renderedCount).toBeGreaterThan(0)
+  })
+})