feat(desktop): add Settings to macOS menus

[rnc505]

Background

• Buzz Settings can be opened from the app UI or keyboard shortcut, but macOS users do not have native Settings entries in either Buzz menu.

Overview of Changes

• Add Settings… below About in the macOS application menu, with the standard Command-, shortcut.
• Add Settings above Quit Buzz in the status-item menu.
• Restore and focus the main window before opening Settings.
• Ignore Settings actions before the logged-in app shell is ready.
• Keep repeated Settings actions idempotent so Back still returns to the previous screen.

Testing Performed

• Full just ci gate; reran desktop Tauri tests with the harness-injected BUZZ_AGENT_PROVIDER unset so the environment-isolation test exercises its intended state.
• Manually verified both native menu placements on macOS.
• Manually verified both actions are no-ops before login.
• Manually verified both actions open Settings after login.
• Manually verified repeated actions keep Settings open without adding duplicate navigation history.

[rnc505]

🤖 ### macOS application menu

Settings appears directly below About and uses the standard Command-, shortcut.

macOS status-item menu

Settings appears immediately above Quit Buzz.

[rnc505]

Application menu

Settings… appears below About Buzz.

Menu bar status item

Settings appears above Quit Buzz.