Preview tool: support multiple tabs and custom URLs

Feature Request

The preview browser is locked to a single dev server port and can't navigate to other localhost URLs (e.g., Docker containers on different ports). It also blocks POST requests to /api paths with CSRF protection (server: granian), making auth-gated apps untestable.

Current Limitations

• Preview browser is locked to the port of the started dev server
• Cannot navigate to other localhost ports (e.g., Docker frontend on 8086 while Vite runs on 5174)
• POST requests to /api paths are intercepted by the preview tool's proxy with CSRF protection, returning 403
• GET requests to proxied paths also don't reach the Vite proxy — they return the SPA HTML instead
• URL bar shows "Link to 127.0.0.1 was blocked. Preview only supports localhost URLs" when trying to navigate

Requested Enhancements

1. Multiple preview tabs for different projects/ports running simultaneously
2. Navigation to arbitrary localhost URLs (not locked to the started server's port)
3. Option to disable CSRF interception for proxied API calls so auth flows work
4. Proper Vite proxy passthrough — let the dev server's proxy config work without interference

Use Case

A Docker Compose project with a frontend (port 8086, nginx) and backend (port 8010, FastAPI). The Vite dev server (port 5174) proxies /api to the backend, but the preview tool intercepts these requests before they reach Vite's proxy. Login is impossible, making the preview unusable for auth-gated applications.

Environment

• Windows 11
• Claude Code CLI
• Vite 5.4 dev server with proxy config
• Docker Compose (nginx frontend + FastAPI backend)