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>

test/simpleshop_theme/theme/presets_test.exs

@@ -42,7 +42,7 @@ defmodule SimpleshopTheme.Theme.PresetsTest do
       assert preset.density == "balanced"
       assert preset.grid_columns == "4"
       assert preset.header_layout == "standard"
-      assert preset.accent_color == "#3b82f6"
+      assert preset.accent_color == "#2563eb"
     end
 
     test "returns boutique preset" do