Add copy-to-clipboard button to code blocks

Injects a hover-revealed copy button into every <pre><code> block
site-wide via a single layout partial, reading textContent so escaped
samples copy as literal text. Ships copy/check icons and matching Sass.

Passes icon markup to JS via <template> elements instead of Twig's
|escape('js'), which calls mb_ord() and fatals without mbstring — same
class of bug as the existing |slice/mb_substr gotcha, now documented in
AGENTS.md.

Closes the "Copy-to-clipboard button on code blocks" backlog item in
novaconium/ISSUES.md.

public/css/main.css

@@ -133,6 +133,7 @@ code {
 }
 
 pre {
+  position: relative;
   background: var(--surface);
   border: 1px solid var(--border-color);
   border-radius: 6px;
@@ -144,6 +145,36 @@ pre code {
   padding: 0;
   color: var(--text-color);
 }
+pre:hover .copy-code-button, pre .copy-code-button:focus-visible {
+  opacity: 1;
+}
+
+.copy-code-button {
+  position: absolute;
+  top: 0.5rem;
+  right: 0.5rem;
+  display: inline-flex;
+  align-items: center;
+  gap: 0.3rem;
+  background: var(--bg);
+  border: 1px solid var(--border-color);
+  border-radius: 4px;
+  color: var(--muted-color);
+  font-size: 0.75rem;
+  padding: 0.25rem 0.5rem;
+  cursor: pointer;
+  opacity: 0;
+  transition: opacity 0.15s ease;
+}
+.copy-code-button:hover {
+  background: var(--bg);
+  color: var(--text-color);
+  border-color: var(--accent);
+}
+.copy-code-button.copied {
+  color: var(--accent);
+  border-color: var(--accent);
+}
 
 hr {
   border: none;