Use the real institutional logo for the header wordmark, not plain text

The header link was just styled "UHHM" text, unrelated to the actual
brand mark - swapped in the institutional logo (the three-arch
"building" mark, matching infrastructure/assets/brand/institutional-*.svg
and Gitea's own homepage logo) instead. Dark-stroke variant only -
portal is a single permanent dark theme, no light mode to switch for.
Same explicit-route treatment the favicon files needed (the /*any
wildcard route would otherwise swallow the request before the static
file handler gets a chance).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app.rs

@@ -368,7 +368,7 @@ fn Hero(title: String, description: String, show_yes: bool) -> impl IntoView {
                 })}
             <div class="hero-copy">
                 <a class="wordmark" href="/">
-                    {SITE_NAME}
+                    <img src="/wordmark.svg" alt=SITE_NAME/>
                 </a>
                 <h1>{title}</h1>
                 <p>{description}</p>