Feature: Support multiple tabs/windows in Preview panel

Feature Request

Problem:
The Preview panel only supports a single dev server at a time. When working with a full-stack project (e.g. Django backend on port 8000 + Vite frontend on port 5174), you can only preview one at a time. Opening the backend debug/admin page requires leaving the Claude Code UI entirely.

Request:

• Support multiple Preview tabs or split-pane previews
• Allow switching between different localhost ports without stopping/restarting the preview server
• Ideally persist the last-used port per project

Use case:
Full-stack development where frontend and backend both have browser UIs that need to be checked simultaneously (e.g. React app + Django admin/debug pages).

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