fix: improve primary button hover state

- Replace filter: brightness with explicit var(--blue2) background
- Avoids unintended text color shifts on hover

Author: Zehee

src/vis/static/style.css

@@ -454,7 +454,7 @@ body {
 }
 
 .btn-primary:hover {
-  filter: brightness(1.15);
+  background: var(--blue2);
 }
 
 .btn-secondary {