macOS menubar: post a system notification when an update is available

[ShunmeiCho]

Today UpdateChecker polls the GitHub releases API every 2 days and surfaces the result only as the "Update" badge inside the popover, the "CLI x.y.z available" banner, and the "Check for Updates" menu item. If I do not open the popover I never learn that a release shipped, and releases land roughly weekly (v0.9.23 on 08-29, v0.9.24 on 09-04). There is no UNUserNotificationCenter usage anywhere under mac/, so nothing proactive reaches the user.

Environment: macOS 26 (Darwin 25.6.0), CodeBurnMenubar 0.9.24, CLI 0.9.24 via npm -g.

Proposal

• When check() detects updateAvailable or cliUpdateAvailable, post one UNUserNotificationCenter notification per new version (deduped with a UserDefaults key so it does not repeat every 2 days).
• Clicking the notification calls the existing performFullUpdate(), so the one-click path stays the single update code path.
• Add a "Notify me about updates" toggle in Settings. Notification permission is requested lazily the first time a notification would be posted. I can default it to off if you prefer no new permission prompt.
• No change to the CLI, the check interval, or the release/install flow.

Scope is contained to UpdateChecker.swift, CodeBurnApp.swift (notification delegate + click handling) and SettingsView.swift, with unit coverage for the dedupe logic under mac/Tests.

I am happy to implement this if the approach is acceptable. Filing here first per CONTRIBUTING.