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/widgets/src/clide_palette.dart

@@ -126,11 +126,11 @@ class _ClidePaletteState extends State<ClidePalette> {
                   color: tokens.dropdownBackground,
                   border: Border.all(color: tokens.dropdownBorder),
                   borderRadius: BorderRadius.circular(6),
-                  boxShadow: const [
+                  boxShadow: [
                     BoxShadow(
-                      color: Color(0x40000000),
+                      color: tokens.shadowAmbient,
                       blurRadius: 12,
-                      offset: Offset(0, 4),
+                      offset: const Offset(0, 4),
                     ),
                   ],
                 ),