fix: support grid-cols-4, accent color, and header layout controls

- Fix grid columns 4 by using conditional logic instead of string interpolation
- Simplify CSS generator to only generate accent colors (mood/typography/shape/density now use data attributes)
- Add header component to all 7 preview pages with logo, nav, and cart
- Add header layout CSS for centered and minimal variants
- Move nav display property to CSS to enable minimal mode hiding

This resolves all high-priority theme control issues:
- Grid columns now supports 2, 3, and 4 columns
- Accent color picker now updates button colors
- Header layout control now works (standard, centered, minimal)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

assets/css/theme-layer2-attributes.css

@@ -155,3 +155,22 @@
 .preview-frame[data-density="compact"] {
   --t-density: 0.85;
 }
+
+/* Header Layout */
+.shop-nav {
+  display: flex;
+}
+
+.preview-frame[data-header="centered"] .shop-header {
+  flex-direction: column;
+  gap: 0.75rem;
+  text-align: center;
+}
+
+.preview-frame[data-header="centered"] .shop-nav {
+  justify-content: center;
+}
+
+.preview-frame[data-header="minimal"] .shop-nav {
+  display: none !important;
+}