refactor: replace inline getByRole('dialog') with page objects in E2E tests (#10724)

## Summary

Replace remaining inline `getByRole('dialog')` calls in E2E tests with
page object accessors, following the pattern from PR #10586.

**Note:** The original scope covered 9 calls across 5 files. Three of
those refactors (ConfirmDialog, MediaLightbox, TemplatesDialog) were
already merged into main via other PRs, so this PR now covers only the
remaining 3 files.

## Changes

- **ComfyNodeSearchBox.ts**: Extract a `root` locator on
`ComfyNodeSearchFilterSelectionPanel` so `header` getter uses
`this.root` instead of an inline `page.getByRole('dialog')`
- **AppModeHelper.ts**: Add `imagePickerPopover` getter that locates the
PrimeVue Popover (`role="dialog"`) filtered by a button named "All"
- **appModeDropdownClipping.spec.ts**: Replace 4-line inline
`getByRole('dialog')` chain with `comfyPage.appMode.imagePickerPopover`

## Review Focus

Pure test infrastructure refactor — no production code changes. Each
page object follows existing conventions.

Fixes #10723

browser_tests/tests/appModeDropdownClipping.spec.ts

@@ -145,10 +145,7 @@ test.describe('App mode dropdown clipping', { tag: '@ui' }, () => {
 
     // The unstyled PrimeVue Popover renders with role="dialog".
     // Locate the one containing the image grid (filter buttons like "All", "Inputs").
-    const popover = comfyPage.page
-      .getByRole('dialog')
-      .filter({ has: comfyPage.page.getByRole('button', { name: 'All' }) })
-      .first()
+    const popover = comfyPage.appMode.imagePickerPopover
     await expect(popover).toBeVisible({ timeout: 5000 })
 
     const isInViewport = await popover.evaluate((el) => {