Home page: the Gitea logo in the header, linking to the source (#216)

The logo is Gitea's own, from git.sdf1.net, with its inline styles
turned into fill attributes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

site/index.html

@@ -15,6 +15,7 @@
   <a class="home" href="/"><img src="/logo.svg" alt="" width="36" height="36">iPX</a>
   <nav aria-label="Elsewhere">
     <a href="https://directory.ipxreader.app">Directory</a>
+    <a class="src" href="https://git.sdf1.net/rays/ipx" title="Source code on git.sdf1.net"><img src="/gitea.svg" alt="Source code" width="28" height="28"></a>
   </nav>
 </header>