wire design palette through resolver with fallback chains

Surface map now tries palette keys (bgSunken, surfaceHi, textHi,
etc.) first, falling back to semantic refs for legacy themes.
Sidebar/panel bg differentiates from workspace bg. Buttons use
accent colour with onAccent text. Four-level text gradient flows
to widgets.

WCAG-AA contrast fixes: clide textDim lightened, midnight
onAccent darkened, paper textDim darkened. Contrast test updated
to cover all four themes. Resolver tests updated for the new
palette shape. Goldens regenerated.

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

app/test/a11y/contrast_test.dart

@@ -11,7 +11,10 @@ void main() {
 
   group('contrast — all bundled themes meet WCAG AA', () {
     const bundledPaths = [
-      'lib/kernel/src/theme/themes/summer-night.yaml',
+      'lib/kernel/src/theme/themes/clide.yaml',
+      'lib/kernel/src/theme/themes/midnight.yaml',
+      'lib/kernel/src/theme/themes/paper.yaml',
+      'lib/kernel/src/theme/themes/terminal.yaml',
     ];
 
     for (final path in bundledPaths) {