fix: improve header navigation accessibility

- Current page in nav is now a span instead of a link (no self-links)
- Logo links to home page, except when already on home
- Use aria-current="page" with accent underline for current page indicator
- Extract logo_content, logo_inner, and nav_item helper components
- Update CSS to target both a and span elements in .shop-nav

This follows WCAG accessibility guidelines - links that point to
the current page are confusing for screen reader users.

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

assets/css/theme-semantic.css

@@ -221,17 +221,19 @@
 }
 
 /* Nav link styling with active state indicator */
-.shop-nav a {
+.shop-nav a,
+.shop-nav span {
   padding: 0.5rem 0;
   border-bottom: 2px solid transparent;
   transition: border-color 0.2s ease, color 0.2s ease;
-
-  &:hover {
-    color: var(--t-text-primary);
-  }
-
-  &[aria-current="page"] {
-    color: var(--t-text-primary);
-    border-bottom-color: hsl(var(--t-accent-h) var(--t-accent-s) var(--t-accent-l));
-  }
+}
+
+.shop-nav a:hover {
+  color: var(--t-text-primary);
+}
+
+.shop-nav a[aria-current="page"],
+.shop-nav span[aria-current="page"] {
+  color: var(--t-text-primary);
+  border-bottom-color: hsl(var(--t-accent-h) var(--t-accent-s) var(--t-accent-l));
 }