fix(linux): detect Chrome/Chromium on Linux and enable safe restart

[DivInstance]

Summary

• Adds Linux support to browser launching:
  • Detect Chrome/Chromium via PATH (google-chrome, google-chrome-stable, chromium, chromium-browser).
  • Adds Linux restart command (pkill -f {browser}).
  • Extends OS registry to include Linux.
  • Improves error message with actionable guidance.

Motivation

• Linux users could not launch the browser automatically, causing failures when harvesting tokens.
• This change directly addresses stability and usability on Linux.

Notes

• No dependency changes required.
• Maintains the minimal, platform-specific branching already used for macOS and Windows.

Closes #143