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>

lib/simpleshop_theme/theme/presets.ex

@@ -23,7 +23,7 @@ defmodule SimpleshopTheme.Theme.Presets do
       density: "balanced",
       grid_columns: "4",
       header_layout: "standard",
-      accent_color: "#3b82f6",
+      accent_color: "#2563eb",
       layout_width: "wide",
       card_shadow: "sm",
       announcement_bar: true