Small fixes: gutter alignment, paste column, sort arrows, copy feedback, stats bar, title slug (#50-#55)

- #50 gutter/code line misalignment: shared --code-lh/--code-fs tokens;
  .code .gutter drops its own vertical padding (was 14px vs 0, 14px offset)
- #51 history Paste column shows only the paste name, or slug pill when untitled
- #52 sort indicator arrow sits LEFT of the column label for all sortable columns
- #53 copy buttons give in-place success feedback ('Success!' in --ok for 2s):
  paste view copy button and new-page result copy button
- #54 collapsed stats summary bar: roomier padding (14px 18px) and item gap (16px);
  wraps gracefully on mobile
- #55 paste view title bar: slug pill only shown for custom slugs, id not repeated

Closes #50, closes #51, closes #52, closes #53, closes #54, closes #55

web/templates/paste.html

@@ -4,10 +4,10 @@
   <div class="float">
     <div class="paste-title-bar">
       <h1>{{if .Title}}{{.Title}}{{else}}Untitled paste{{end}}</h1>
-      <span class="slug">/{{if .CustomSlug}}{{.CustomSlug}}{{else}}{{.ID}}{{end}}</span>
+      {{if .CustomSlug}}<span class="slug">/{{.CustomSlug}}</span>{{end}}
       <div class="spacer"></div>
       <a class="iconbtn" href="/raw/{{.ID}}">raw</a>
-      <a class="iconbtn" href="#" onclick="copyContent(); return false;">copy</a>
+      <a class="iconbtn" href="#" id="copy-btn" onclick="copyContent(this); return false;">copy</a>
       {{if .DeletionToken}}<a class="iconbtn danger" href="#" onclick="redeem('{{.DeletionToken}}'); return false;">delete</a>{{end}}
     </div>
   </div>
@@ -63,9 +63,17 @@ function toggleStats() {
   pill.classList.toggle('open', open);
   btn.setAttribute('aria-expanded', open ? 'true' : 'false');
 }
-function copyContent() {
+function copyContent(btn) {
   navigator.clipboard.writeText(document.getElementById('raw-content').value);
-  toast('Copied');
+  // in-place success feedback (#53)
+  if (btn) {
+    btn.classList.add('ok');
+    btn.textContent = 'Success!';
+    clearTimeout(btn._okh);
+    btn._okh = setTimeout(() => { btn.classList.remove('ok'); btn.textContent = 'copy'; }, 2000);
+  } else {
+    toast('Copied', 'success');
+  }
 }
 function redeem(token) {
   if (!confirm('Hard delete this paste immediately?')) return;