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>

lib/simpleshop_theme_web/live/shop_live/collection.ex

@@ -100,7 +100,7 @@ defmodule SimpleshopThemeWeb.ShopLive.Collection do
   @impl true
   def render(assigns) do
     ~H"""
-    <div class="shop-container min-h-screen" style="background-color: var(--t-surface-base); font-family: var(--t-font-body); color: var(--t-text-primary);">
+    <div class="shop-container min-h-screen pb-20 md:pb-0" style="background-color: var(--t-surface-base); font-family: var(--t-font-body); color: var(--t-text-primary);">
       <SimpleshopThemeWeb.ShopComponents.skip_link />
 
       <%= if @theme_settings.announcement_bar do %>
@@ -158,6 +158,8 @@ defmodule SimpleshopThemeWeb.ShopLive.Collection do
       <SimpleshopThemeWeb.ShopComponents.cart_drawer cart_items={@cart_items} subtotal={@cart_subtotal} mode={@mode} />
 
       <SimpleshopThemeWeb.ShopComponents.search_modal hint_text={~s(Try searching for "mountain", "forest", or "ocean")} />
+
+      <SimpleshopThemeWeb.ShopComponents.mobile_bottom_nav active_page="collection" mode={@mode} />
     </div>
     """
   end