Improve shop footer: add description, nav links, feeds, account

Shop footer now has 4-column grid matching SaaS layout with shop
description, cart/subscribe links, RSS/Atom feeds, and account link.

make_post_sell/templates/snippets/footer.j2

@@ -49,11 +49,34 @@
 
     <div class="mps-footer-brand">
       <a href="/" class="mps-footer-logo">{{ request.shop.name }}</a>
+      {% if request.shop.description %}
+      <p class="mps-footer-tagline">{{ request.shop.description }}</p>
+      {% endif %}
     </div>
 
     <div class="mps-footer-group">
+      <span class="mps-footer-group-title">{{ request.shop.name }}</span>
       <nav class="mps-footer-links">
         <a href="/">Home</a>
+        <a href="/cart">Cart</a>
+        {% if request.shop.subscriptions_enabled %}
+        <a href="/subscribe">Subscribe</a>
+        {% endif %}
+      </nav>
+    </div>
+
+    <div class="mps-footer-group">
+      <span class="mps-footer-group-title">Feeds</span>
+      <nav class="mps-footer-links">
+        <a href="/rss.xml">RSS</a>
+        <a href="/atom.xml">Atom</a>
+      </nav>
+    </div>
+
+    <div class="mps-footer-group">
+      <span class="mps-footer-group-title">Account</span>
+      <nav class="mps-footer-links">
+        <a href="/u/settings">My Account</a>
         <a href="https://makepostsell.com" target="_blank">Powered by makepostsell.com</a>
       </nav>
     </div>