Fix z-index conflict between header/footer and pattern icons

Bump header and footer from z-10 to z-20 so they stay above
pattern card favorite icons when scrolling.

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

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

templates/base.html

@@ -262,7 +262,7 @@
     >
       <div class="layout-container flex min-h-full grow flex-col">
         <header
-          class="fixed top-0 left-0 right-0 z-10 flex items-center justify-between whitespace-nowrap border-b border-solid border-b-gray-200 bg-white px-4 sm:px-6 md:px-10 py-3 sm:py-4 shadow-sm"
+          class="fixed top-0 left-0 right-0 z-20 flex items-center justify-between whitespace-nowrap border-b border-solid border-b-gray-200 bg-white px-4 sm:px-6 md:px-10 py-3 sm:py-4 shadow-sm"
         >
           <div class="flex items-center gap-3 text-gray-800">
             <a href="/" class="flex items-center gap-3 text-gray-800 hover:opacity-80 transition-opacity">
@@ -339,7 +339,7 @@
         </button>
 
         <footer
-          class="fixed bottom-0 left-0 right-0 z-10 bg-white border-t border-gray-200 shadow-t-sm"
+          class="fixed bottom-0 left-0 right-0 z-20 bg-white border-t border-gray-200 shadow-t-sm"
         >
           <nav class="mx-auto flex max-w-5xl justify-around">
             <a