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.

novaconium/pages/admin/docs/styling/index.twig

@@ -57,4 +57,8 @@ docker run --rm -v "$(pwd):/usr/src/app" -w /usr/src/app novaconium-sass \
     <p>The toggle button in <code>novaconium/pages/_layout/nav.twig</code> flips a <code>data-theme</code> attribute on <code>&lt;html&gt;</code> at runtime and persists the choice to <code>localStorage</code>. <code>novaconium/pages/_layout/theme-init.twig</code>, included early in <code>&lt;head&gt;</code> before the stylesheet, re-applies a saved choice before first paint on every later page load, so switching to light doesn't flash dark first. The sun/moon icon swap inside the button is pure CSS reacting to the attribute — no JS involved there — so it works correctly even on sidecar-less pages that get statically cached.</p>
 
     <p>To customize the light theme the same way you'd customize the dark one, edit the <code>-light</code> variables in <code>App/sass/_colors.sass</code> and recompile.</p>
+
+    <h2>Copy-to-clipboard on code blocks</h2>
+
+    <p><code>novaconium/pages/_layout/code-copy.twig</code>, included once from <code>_layout/layout.twig</code>'s footer, injects a hover-revealed copy button into every <code>&lt;pre&gt;</code> containing a <code>&lt;code&gt;</code> on the page — no per-page markup needed. It copies via <code>code.textContent</code> (not <code>innerHTML</code>), so HTML-entity-escaped samples like <code>&amp;lt;h1&amp;gt;</code> in the <a href="/admin/docs/seo">SEO</a> starter template come out as literal characters, not escaped markup. Uses the same event-delegation pattern as the theme toggle in <code>_layout/nav.twig</code>: one document-level click listener rather than a listener per button.</p>
 {% endblock %}