feat: add toggle to show/hide menu bar icon

[Karl-Dai]

Summary

• Adds a "Show Menu Bar Icon" toggle in Settings → General (default on), persisted in SQLite alongside other settings.
• When the icon is hidden, global shortcuts (cmd+shift+v, F1, F3) continue to work; re-launching the app opens Preferences via DistributedNotificationCenter signaling between the second-launch and running instance.
• Cold-start fallback: if the icon is hidden and the app starts fresh, Settings opens automatically once AppSettings finishes its async DB load (deterministic poll on a new isLoaded signal, 5s timeout).

Architecture notes

• Replaced WindowGroup(id: "settings") with the standard SwiftUI Settings { … } scene so Settings can be opened from AppDelegate via NSApp.sendAction(Selector(("showSettingsWindow:"))) regardless of whether the window is currently alive.
• MenuBarExtra uses isInserted: $settings.showMenuBarIcon for live show/hide.
• Notification name com.antigravity.veloxclip.openSettings; observer body wrapped in Task { @MainActor in … } for Swift 6 strict-concurrency correctness.

Files changed

• VeloxClip/Models/AppSettings.swift — showMenuBarIcon property + isLoaded signal + DB persistence.
• VeloxClip/Views/SettingsView.swift — toggle in General section.
• VeloxClip/App/VeloxClipApp.swift — Settings scene, MenuBarExtra binding, distributed-notification post + observer, cold-start fallback.
• docs/superpowers/specs/2026-05-07-menu-bar-icon-toggle-design.md — design.
• docs/superpowers/plans/2026-05-07-menu-bar-icon-toggle.md — implementation plan (incl. documented refinement vs. spec).

Test plan

This repo has no automated test target (VeloxClipTests declaration in Package.swift is a stub; no Tests/ directory exists). Manual verification per the spec checklist:

• Fresh install → menu bar icon visible.
• Toggle off → icon disappears immediately; cmd+shift+v / F1 / F3 still work.
• With icon hidden, double-click VeloxClip.app → Settings opens, app activates.
• With icon hidden, cmd+Q then re-launch from Spotlight → Settings opens automatically (cold-start fallback).
• Toggle back on while Settings is open → icon reappears immediately.
• Reboot macOS, launch app → toggle state persists.
• With icon visible and Settings closed, double-click VeloxClip.app → Settings still opens (verifies the duplicate-launch signal path).