Add responsive mobile layouts and navigation

src/lib/ui/components/primitives/Button.svelte

@@ -72,6 +72,7 @@
 
 <style>
     .btn {
+        max-width: 100%;
         display: inline-flex;
         align-items: center;
         justify-content: center;
@@ -159,4 +160,7 @@
         animation: spin 0.7s linear infinite;
     }
     @keyframes spin { to { transform: rotate(360deg); } }
+    @media (max-width: 720px) {
+        .btn { white-space: normal; line-height: 1.35; }
+    }
 </style>