tokenise window-control colours + palette shadow

Hard-coded Windows close-button red (#E81123), white close glyph, and
the palette's 0x40000000 drop shadow were the three colour sites the
UX consultant flagged as not adapting per theme. Now they're
`windowControl.closeHoverBackground` / `closeHoverForeground` /
`shadow.ambient`. Paper themes override the shadow to a softer ink so
it doesn't read as a CRT halo on cream.

Co-Authored-By: Claude <noreply@anthropic.com>

lib/kernel/src/theme/themes/paper-hc.yaml

@@ -28,6 +28,9 @@ palette:
   textMute: "#7A7A72"
   accentSoft: "#218E2D10"
 
+surface:
+  shadow.ambient: "#26000000"
+
 syntax:
   keyword: "#52215C"
   type: "#154168"

lib/kernel/src/theme/themes/paper.yaml

@@ -25,6 +25,9 @@ palette:
   textMute: "#A8A89E"
   accentSoft: "#21C14B2A"
 
+surface:
+  shadow.ambient: "#26000000"
+
 syntax:
   keyword: "#7B3F8C"
   type: "#2A6FC1"