Use semi-transparent accent fill on nav button hover

color-mix(accent 18%, transparent) keeps text readable and gives a
ghost-button tint instead of a full solid fill.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/public/dashboard.html

@@ -41,9 +41,9 @@ a { color: var(--cool); text-decoration: none; }
   transition: background .15s, border-color .15s, color .15s;
 }
 .logout:hover {
-  background: var(--accent);
+  background: color-mix(in srgb, var(--accent) 18%, transparent);
   border-color: var(--accent);
-  color: #fff;
+  color: var(--accent);
 }
 
 /* ── Wrapper ── */