Open in IDE button shows unhelpful 'Unable to open file.' error when no IDE extension is installed

Description

When clicking the Open in IDE button in the Claude Code desktop app, the error message "Unable to open file." is displayed when no supported IDE (VS Code or Cursor) extension is installed. This is confusing and unhelpful.

Expected Behavior

The button should either:

• Be hidden or disabled when no supported IDE is detected, OR
• Show a descriptive message such as: "Install the Claude Code extension for VS Code or Cursor to use this feature."

Actual Behavior

Generic error: "Unable to open file." — no indication that a VS Code/Cursor extension is required.

Environment

• Claude Code version: 2.1.59
• Ghostty version: 1.2.3
• macOS version: Darwin 25.2.0
• Terminal: Ghostty (no VS Code or Cursor installed)

Steps to Reproduce

1. Open Claude Code desktop app in Ghostty (no VS Code/Cursor extension installed)
2. Click the Open in IDE button in the desktop app header
3. See error: "Unable to open file."

Suggested Fix

Detect whether a supported IDE extension is active before showing/enabling the button. If none is detected, show a helpful onboarding message pointing users to install the extension.