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>

app/tags/page.tsx

@@ -21,11 +21,9 @@ export default function TagIndexPage() {
   const topTags = tags.slice(0, 3);
 
   const colorClasses = [
-    'from-rose-400/70 to-rose-200/40',
-    'from-emerald-400/70 to-emerald-200/40',
-    'from-sky-400/70 to-sky-200/40',
-    'from-amber-400/70 to-amber-200/40',
-    'from-violet-400/70 to-violet-200/40'
+    'from-accent/60 to-accent/20',
+    'from-accent/50 to-accent/15',
+    'from-accent/40 to-accent/10',
   ];
 
   // CollectionPage schema with ItemList