fix: unify color system around configurable accent, warm-tint neutrals

Replace hardcoded purple gradients with accent-derived colors so
changing NEXT_PUBLIC_COLOR_ACCENT actually controls the entire site.
Warm-tint ink colors and body background from slate to stone.
Remove decorative floating orbs from hero. Simplify tag page to
accent-derived tints instead of 5 random colors.

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

components/post-list-item.tsx

@@ -21,7 +21,7 @@ export function PostListItem({ post, priority = false }: Props) {
 
   return (
     <article className="motion-card group relative flex gap-4 rounded-2xl border border-white/40 bg-white/60 p-5 shadow-lg backdrop-blur-md transition-all hover:scale-[1.01] hover:shadow-xl dark:border-white/10 dark:bg-slate-900/60">
-      <div className="pointer-events-none absolute inset-x-0 top-0 h-0.5 origin-left scale-x-0 bg-gradient-to-r from-[rgba(124,58,237,0.9)] via-[rgba(167,139,250,0.9)] to-[rgba(14,165,233,0.8)] opacity-80 transition-transform duration-300 ease-out group-hover:scale-x-100" />
+      <div className="pointer-events-none absolute inset-x-0 top-0 h-0.5 origin-left scale-x-0 bg-accent opacity-80 transition-transform duration-300 ease-out group-hover:scale-x-100" />
       {cover && (
         <div className="relative flex h-24 w-24 flex-none overflow-hidden rounded-md bg-slate-100 dark:bg-slate-800 sm:h-auto sm:w-40">
           <Image