fix: improve accent color contrast for WCAG AA compliance

Add WCAG AA compliant accent color variants and update default accent
to meet 4.5:1 contrast ratio requirements.

- Add --t-accent-text (darker for text on light backgrounds)
- Add --t-accent-button (darker for button backgrounds with white text)
- Change default accent from #3b82f6 to #2563eb (better contrast)
- Update presets and tests for new default

These changes ensure accent colors meet accessibility standards while
maintaining visual consistency with the brand palette.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

assets/css/theme-semantic.css

@@ -23,6 +23,12 @@
   --t-accent-subtle: hsl(var(--t-accent-h) 40% 95%);
   --t-accent-ring: hsl(var(--t-accent-h) var(--t-accent-s) var(--t-accent-l) / 0.4);
 
+  /* WCAG AA compliant accent variants for better contrast */
+  /* Darker accent for text on light backgrounds (4.5:1 with white) */
+  --t-accent-text: hsl(var(--t-accent-h) var(--t-accent-s) 38%);
+  /* Darker accent for button backgrounds to ensure 4.5:1 with white text */
+  --t-accent-button: hsl(var(--t-accent-h) var(--t-accent-s) 42%);
+
   /* Secondary colors */
   --t-secondary-accent: #ea580c;
   --t-sale-color: #dc2626;