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/fixtures/helpers/AppModeHelper.ts

@@ -83,6 +83,14 @@ export class AppModeHelper {
     return this.page.locator('[data-testid="linear-widgets"]')
   }
 
+  /** The PrimeVue Popover for the image picker (renders with role="dialog"). */
+  get imagePickerPopover(): Locator {
+    return this.page
+      .getByRole('dialog')
+      .filter({ has: this.page.getByRole('button', { name: 'All' }) })
+      .first()
+  }
+
   /**
    * Get the actions menu trigger for a widget in the app mode widget list.
    * @param widgetName Text shown in the widget label (e.g. "seed").