fix(button): remove color override on anchor buttons

`a.btn` had `color: inherit` which overrode variant-specific colors
(e.g. `--fg-on-accent` on `.btn-primary`) due to higher specificity
(element+class > class). This caused black text on primary link buttons
in light mode. Let variant classes control text color instead.

src/ui/button.css

@@ -61,11 +61,9 @@
 /* Link buttons rendered as <a> need reset styles */
 a.btn {
   text-decoration: none;
-  color: inherit;
 }
 a.btn-link {
   text-decoration: underline;
-  color: var(--accent);
 }
 
 .btn-sm {