Preview tool: support multiple tabs/viewports for side-by-side comparison

Feature Request

Problem

When comparing two pages side-by-side (e.g., verifying button consistency between Survey list and Reward detail, or checking responsive layout at different breakpoints), the current Preview tool only supports a single viewport per server. You have to navigate back and forth between pages, which makes visual comparison extremely tedious and error-prone.

Proposed Solution

Allow the Preview tool to open multiple tabs or viewports for the same dev server, so users can view different pages simultaneously.

Option A — Multiple tabs per server:

Option B — Side-by-side composite screenshot:

Use Cases

1. Cross-page consistency — compare button sizes, typography, spacing between different pages
2. Responsive testing — same page at 1280px vs 1536px side by side
3. Before/after — compare a page before and after a CSS change
4. Detail vs list — verify that list page and detail page use consistent patterns

Current Workaround

Navigate between pages one at a time, take screenshots, and mentally compare. This is slow and unreliable for subtle visual differences like spacing, font weight, or icon sizes.