titlebar: move tabs and sidebar toggle into title bar

[junereycasuga]

Closes: #702

Adds a "Tabs in title bar" setting that puts the note tab strip and sidebar toggle in the title bar, replacing the centered window title.

• Off by default.
• Only active when one pane is open; split layouts fall back to inline tabs and the centered title.
• Adds a titlebarTabs pref and renders the active EditorPane tab strip into a host inside TitleBar via portal.
• Hides the duplicate sidebar toggle in the Sidebar header while active.
• Preserves macOS traffic-light space and Windows/Linux window controls.

Screenshots: