feat: enable native browser launching in WSL with GUI support

[Seismix]

Overview

This PR adds support for launching browsers in WSL environments with GUI capabilities (e.g. WSLg).

Changes:

• Updated WSL runner logic to detect GUI availability and use native browser launching when available
• Added browser installation checks with helpful error messages when browsers are not found
• Added tests for WSL scenarios:
  • X11 and Wayland environments
  • WSL without GUI (shows manual install warning)
  • Non-WSL environments (no behavior change)
  • Missing browser installations (helpful error messages)

Manual Testing

Prerequisites:

• WSL2 with GUI support (WSLg on Windows 11, or X11/Wayland server with DISPLAY/WAYLAND_DISPLAY set)
• Firefox or Chrome installed in WSL

Test steps:

1. Follow the setup instructions in CONTRIBUTING.md
2. Start the demo extension: cd packages/wxt-demo && pnpm dev:firefox (or pnpm dev for Chrome)
3. Verify browser opens automatically with extension loaded

Expected results:

• Browser should open automatically in WSL with GUI
• Extension should load correctly
• No "Cannot open browser" warnings

Related Issue

This PR closes #1931

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