fix(desktop): open external links in system browser instead of webview

[turculaurentiu91]

Summary

• External links in markdown content (AI responses) now open in the system's default browser instead of inside the Tauri webview

Problem

When clicking links in AI-generated markdown output in the desktop app, such as GitHub PR links, they were opening inside the Tauri webview rather than in the system's default browser. This happened because target="_blank" links bypass Tauri's on_navigation handler.

Solution

• Changed the markdown link renderer to use class="external-link"
• Added a global click listener in the desktop entry point that intercepts clicks on .external-link anchors and opens them via platform.openLink() (which uses Tauri's shell plugin)

This keeps the fix isolated to the desktop package while reusing the existing openLink platform API.