Create a Page Object for the SelectionToolbox in browser tests

## Summary

The `SelectionToolbox` component currently lacks a dedicated Playwright Page Object in the browser test suite. Tests in `browser_tests/tests/selectionToolboxMoreActions.spec.ts` (and related specs) interact with the toolbox via raw locators (e.g., `.selection-toolbox` class selectors and `getByTestId` calls scattered throughout the spec files).

A dedicated Page Object (e.g., `SelectionToolboxPage` or similar, under `browser_tests/fixtures/`) would:
- Encapsulate all locator logic for the toolbox and its buttons (more-options button, refresh button, bypass button, etc.)
- Provide typed helper methods for common actions (open more options, click pin/unpin, align, distribute, etc.)
- Improve maintainability and readability across all toolbox-related specs

## Requested by

Requested by @DrJKL in PR #10968 (comment: https://github.com/Comfy-Org/ComfyUI_frontend/pull/10968#discussion_r3075918232).

## References

- PR: https://github.com/Comfy-Org/ComfyUI_frontend/pull/10968

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