feat: add mobile bottom navigation bar

Replace cramped horizontal nav on mobile with a fixed bottom tab bar
for thumb-friendly navigation. The header nav is now hidden on mobile
(<768px) and the bottom nav provides Home, Shop, About, and Contact
links with icons.

- Add mobile_bottom_nav component with icon + label nav items
- Active page has accent-colored background highlight and larger icon
- Add shadow to lift nav visually off the page
- Update all page templates with bottom padding and bottom nav
- Remove CSS rule that was overriding Tailwind's hidden class
- Responsive header padding (tighter on mobile)

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

assets/css/theme-layer2-attributes.css

@@ -413,10 +413,8 @@
   }
 }
 
-/* Shop nav display */
-.shop-nav {
-  display: flex;
-}
+/* Shop nav display - hidden on mobile, flex on md+ (via Tailwind classes in component) */
+/* Note: Removed explicit display:flex here as it overrides Tailwind's hidden class */
 
 /* =============================================
    STANDALONE COMPONENTS (Context-independent)