Add floating traffic-light window chrome

Introduces a dedicated layer-shell `WindowChromeSurface` that renders macOS-style traffic-light controls over the focused Sway window titlebar, replacing the old top-bar window controls. Adds theme tokens and a reusable `TrafficLightButton` component, updates shell QML/CMake wiring, and adjusts Sway titlebar padding/title format so window titles do not overlap the overlay. Window tracking was extended with focused-window geometry/deco/fullscreen properties plus backend refresh support so the chrome follows window movement and state changes in near real time.

compositor/sway/nebula-sway.conf

@@ -46,17 +46,20 @@ focus_follows_mouse yes
 # Sway draws a restrained title bar and border on normal application windows
 # so they can be identified, dragged, and resized with the mouse.
 #
-# This is temporary infrastructure. Nebula does not yet own client/server-side
-# decorations. Do not treat these colours or metrics as the final design.
+# Nebula overlays traffic-light controls on the focused window title bar.
+# Horizontal padding keeps the Sway title text clear of those buttons.
+# Menu-style chrome (application name, future File/Edit) stays in the top bar.
 #
-# Layer-shell surfaces (desktop, top bar, launcher) are not xdg_shell/XWayland
-# views and are not decorated by these settings.
+# This is temporary infrastructure until Nebula owns server-side decorations.
+# Layer-shell surfaces (desktop, top bar, launcher, window chrome) are not
+# xdg_shell/XWayland views and are not decorated by these settings.
 font pango:Ubuntu 10
-titlebar_padding 7 3
+titlebar_padding 12 8
 titlebar_border_thickness 1
 default_border normal 1
 default_floating_border normal 1
 hide_edge_borders none
+title_format "                 %title"
 
 # class                 border    backgr.   text      indicator child_border
 client.focused          #3d4d62   #1a222e   #e8ecf2   #7ea0c4   #3d4d62