[Preview] Support multiple tabs for navigating different pages simultaneously

Problem

The built-in preview window can only display one page at a time. When working on multi-page projects (websites, web apps, dashboards), developers constantly need to switch between different routes to verify changes, compare layouts, or test navigation flows. Currently, this requires either:

• Manually changing the URL in the preview each time, or
• Opening an external browser alongside the app — which defeats the purpose of the integrated preview.

Current Behavior

• Preview shows a single page view with no tab system.
• Navigating to a different route replaces the current view entirely.
• No way to keep multiple pages open for quick reference or comparison.

Expected Behavior

A tabbed interface within the preview window, similar to how browsers work:

• Open new tab: A + button to open a new tab pointing to a different route.
• Switch between tabs: Click on tab headers to switch views instantly.
• Close tabs: An × button on each tab to close it.
• Tab labels: Show the route path (e.g., /about, /dashboard, /products/123) as the tab label.

Why This Matters

• Multi-page workflows are the norm: Websites and apps have multiple pages — header changes affect all pages, layout components are shared, navigation must be tested end-to-end.
• Side-by-side comparison: Developers often need to compare the same component across different pages (e.g., "Does the header look correct on both the home page and the course page?").
• Reduced context switching: The more tools are consolidated within the Claude Code preview, the faster and smoother the development workflow becomes.
• Real-world scenario: When building a site with 30+ pages (courses, blog, institutional pages), constantly re-navigating in a single-tab preview is extremely slow and error-prone.

Suggested Implementation

• A tab bar at the top of the preview viewport (below the URL/toolbar area).
• Ctrl+T / Cmd+T to open a new tab.
• Ctrl+W / Cmd+W to close the current tab.
• Tabs persist across HMR reloads (they keep their routes).
• Optional: drag-and-drop tab reordering.

Environment

• Claude Code Desktop App (Windows 11)
• Daily professional use for multi-page frontend development (Next.js App Router)