fix: compute --on-accent in inline head script for all pages

--on-accent was only set by deferred theme-core.js module, causing
all fab--primary and btn--primary elements to have wrong text/icon
color on first paint. Now computed from HSL values in the synchronous
theme-init-script partial, matching theme-core.js logic exactly.

Remove hardcoded fallback from fab--label CSS.

public/css/main.css

@@ -964,12 +964,6 @@ a.fab--secondary:visited {
   width: auto;
   padding: 0.65rem 1.1rem;
   gap: 0.5rem;
-  color: var(--on-accent, hsl(0 0% 98%));
-}
-
-.fab--label:hover,
-.fab--label:focus-visible {
-  color: var(--on-accent, hsl(0 0% 98%));
 }
 
 .fab__label {