feat(ui): add settings link to Open In dropdown

[kessenma]

Summary

• Adds a "Change default..." menu item to the bottom of the "Open in" dropdown (separated by a divider)
• Clicking it opens Settings > General where the Editor, Terminal, and Open In preferences live
• Improves discoverability — new users seeing "Open in Zed" can immediately find how to switch to VS Code, Cursor, etc.

Motivation

The "Open in" button defaults to whatever editor is configured in settings, but there's no indication from the button itself how to change it. Users unfamiliar with the settings layout may not realize they can switch editors. This mirrors the existing "Change defaults in Settings" link already present in the OpenInModal component.

Changes

File	Change
src/components/open-in/OpenInButton.tsx	Add DropdownMenuSeparator + "Change default..." item that calls openPreferencesPane('general')

Test plan

• Click the dropdown chevron on the "Open in" button
• Verify "Change default..." appears below a separator after the existing options
• Click it — Settings dialog should open on the General pane
• Scroll to the Editor/Terminal/Open In fields and verify they're visible

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