fix: reset heading margins and button chrome from semantic HTML changes

The WCAG pass changed <div> to <h3> for dock headers and <span> to
<button> for the zoom reset label. Browser defaults for those elements
(heading margins, button border/background) were never overridden,
causing bloated headers and a Windows-XP-style zoom button.

src/styles/main.css

@@ -52,6 +52,7 @@
   --treeNode:rgba(200,212,238,.40);
 }
 *{box-sizing:border-box;}
+h1,h2,h3{margin:0; font-size:inherit; font-weight:inherit;}
 html,body{height:100%;}
 body{
   margin:0; padding:0; font-family:var(--sans); color:var(--text); overflow:hidden;
@@ -278,6 +279,8 @@ body{
   opacity:.9;
   cursor:pointer;
   padding:4px 6px;
+  border:none;
+  background:transparent;
   border-radius:var(--r3);
   transition:background .15s ease, opacity .15s ease, transform .15s ease;
 }