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>

style/main.css

@@ -97,12 +97,17 @@ main.loading, main.not-found {
 }
 
 .wordmark {
-  font-family: var(--heading);
-  font-size: 0.85rem;
-  letter-spacing: 0.16em;
-  text-transform: uppercase;
-  color: var(--ink-dim);
-  text-decoration: none;
+  display: inline-flex;
+  opacity: 0.85;
+  transition: opacity 120ms;
+}
+
+.wordmark:hover { opacity: 1; }
+
+.wordmark img {
+  height: 2.1rem;
+  width: auto;
+  display: block;
 }
 
 .hero-copy h1 { font-size: 2.1rem; }