Add GitHub social link to palette topbar

Add a 'GitHub' anchor after the existing Git link in the topbar nav,
reusing the inline stroke-based .ext external-link SVG icon so no CSS
changes are needed. href is a placeholder (https://github.com/poslop)
pending Caleb's real social link.

internal/web/templates/layout.html

@@ -14,6 +14,7 @@
       <a href="/public" {{if eq .Page "public"}}class="on"{{end}}>Public</a>
       <a href="/saved" {{if eq .Page "saved"}}class="on"{{end}}>Saved</a>
       <a href="https://git.archfox.org/poslop/palette" target="_blank" rel="noopener">Git<svg class="ext" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M18 13v6a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2V8a2 2 0 0 1 2-2h6"/><polyline points="15 3 21 3 21 9"/><line x1="10" y1="14" x2="21" y2="3"/></svg></a>
+      <a href="https://github.com/poslop" target="_blank" rel="noopener noreferrer" aria-label="GitHub (opens in a new tab)">GitHub<svg class="ext" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M18 13v6a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2V8a2 2 0 0 1 2-2h6"/><polyline points="15 3 21 3 21 9"/><line x1="10" y1="14" x2="21" y2="3"/></svg></a>
     </nav>
     <div class="spacer"></div>
     <button class="iconbtn dark-toggle" id="dark-toggle" type="button" title="Toggle dark mode" aria-label="Toggle dark mode">