docs: add repo link

site/index.html

···
     <div class="app">
       <header class="header">
         <a class="brand" href="./">zat.dev</a>
+        <a
+          class="header-link"
+          href="https://tangled.org/zzstoatzz.io/zat"
+          target="_blank"
+          rel="noopener noreferrer"
+        >
+          repo
+        </a>
       </header>
 
       <div class="layout">

site/style.css

···
   text-decoration: none;
 }
 
+.header-link {
+  margin-left: auto;
+  padding: 8px 10px;
+  border-radius: 10px;
+  border: 1px solid var(--border);
+  color: var(--text);
+  opacity: 0.9;
+}
+.header-link:hover {
+  background: color-mix(in srgb, var(--codebg) 70%, transparent);
+  text-decoration: none;
+  opacity: 1;
+}
+
 .layout {
   display: grid;
   grid-template-columns: 280px 1fr;