Add launchpad.enabled setting to control launchpad visibility

[zambetti]

Goal

This PR makes launchpad visibility explicitly user-configurable through launchpad.enabled.
The goal is to ensure that when users disable launchpad, it is not shown automatically in any startup or empty-workspace fallback path.

Related

Discussion: #47805
Issue: #47075 Requires Repro/Verification

Summary

• Added a new workspace setting: launchpad.enabled (boolean, default true).
• Updated startup logic so automatic launchpad/onboarding display is gated by launchpad.enabled.
• Updated empty-pane welcome fallback (after closing last tab) to also respect launchpad.enabled.
• Updated VS Code import struct construction with launchpad: None for WorkspaceSettingsContent completeness.

Self-Review Checklist

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Release Notes:

• Improved launchpad startup behavior by adding launchpad.enabled and applying it consistently across automatic startup and welcome fallback paths.