feat(ui): hover copy button on markdown code blocks

web/css/copilot-messages.css

@@ -440,6 +440,45 @@
 
 .copilot-markdown pre code { background: none; padding: 0; font-size: inherit; }
 
+/* Fenced code blocks carry a copy button that appears on hover (renderMarkdown
+   wraps every <pre> in .md-code-wrap). */
+.md-code-wrap { position: relative; }
+
+.md-code-copy {
+  position: absolute;
+  top: 0.4rem;
+  right: 0.4rem;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  width: 1.6rem;
+  height: 1.6rem;
+  padding: 0;
+  border: none;
+  border-radius: 0.35rem;
+  background: rgba(0, 0, 0, 0.12);
+  color: inherit;
+  font-size: 0.8rem;
+  line-height: 1;
+  cursor: pointer;
+  opacity: 0;
+  transition: opacity 0.15s ease;
+}
+
+.md-code-wrap:hover .md-code-copy,
+.md-code-copy:focus-visible { opacity: 1; }
+
+.md-code-copy.copied { opacity: 1; color: #16a34a; }
+
+@media (prefers-color-scheme: dark) {
+  .md-code-copy { background: rgba(255, 255, 255, 0.14); }
+}
+
+/* No hover on touch: keep the button dimly visible so it stays reachable. */
+@media (hover: none) {
+  .md-code-copy { opacity: 0.55; }
+}
+
 .copilot-markdown blockquote {
   border-left: 3px solid var(--accent);
   margin: 0.5rem 0;