Light-scheme contrast, borders, and an opaque window

Control borders and surface separation come back on light schemes, hover states stay legible, swatches are labelled, and the window gets an opaque backing surface on Windows so a transparent frame does not show the desktop through the chrome.

Tabby's light schemes lost control borders and surface separation, hover states went illegible, colour swatches had no labels, and on Windows the window had no opaque backing surface — so a transparent frame showed the desktop through the chrome.

Borders and separation restored on light schemes, hover contrast fixed, swatches labelled, tooltips added where a control said nothing, an opaque backing colour applied on Windows, and the dark-mode active tab given real contrast — with the active-tab bar dropped in vertical bars, where it duplicates the selection it sits inside.

• setDarkMode had to be guarded against an unset vibrancy state, which threw during window construction — a fix for the fix.