#441 fix(desktop): use user's preferred app for terminal click-to-open

Summary

• Terminal file path clicks now open files in the user's preferred app (from OpenInButton selection) instead of hardcoding Cursor/VS Code
• Refactored external router to extract pure helper functions and eliminate duplication
• Added 24 tests for helper functions

Test plan

• Typecheck passes
• 24 new tests pass for helpers
• Manual: Set preferred app via OpenInButton dropdown, then Cmd+click a file path in terminal - should open in that app

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features

  • Enhanced support for opening files across additional applications (VS Code, Sublime, Xcode, JetBrains IDEs, terminal emulators, and others).
  • Improved file path handling including home directory expansion and relative path resolution.

• Tests

  • Added comprehensive test coverage for external application integration and path resolution utilities.

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