[mac] Add View → Hide Toolbar (⌥⌘T) for focused writing

[Shpigford]

Summary

• Adds a custom View → Hide Toolbar menu item (⌥⌘T) that flips window.toolbar?.isVisible from ClearlyAppDelegate, persisted in UserDefaults("toolbarHidden").
• When the toolbar is hidden, traffic lights also fade out and reappear via a NSEvent mouseMoved monitor when the cursor enters the top ~50pt of the window — a true focus-mode chrome hide.
• Scrubs AppKit's auto-injected "Hide Toolbar" / "Customize Toolbar…" items so they don't race with the custom action; filters the Settings window from the main-window lookup so its toolbar isn't accidentally affected; tracks window-identity changes so a close+reopen of the document window restores hidden-mode state on the new NSWindow.

Fixes #287

Test plan

• Press ⌥⌘T → toolbar items + plate collapse, traffic lights fade out
• Move cursor toward the top of the window → traffic lights fade in; move down → fade out
• ⌥⌘T again → full chrome restored, lights stay solid
• Quit + relaunch with toolbar hidden → still hidden on launch, menu reads "Show Toolbar"
• All keyboard shortcuts still work in hidden mode (⌘1, ⌘2, ⇧⌘O, ⇧⌘B, ⌘F, ⌘P, ⇧⌘F)
• Open Settings (⌘,), then ⌥⌘T from main window → only main toolbar toggles, Settings chrome unaffected