Smoother tab switch (no dark flash), full welcome sentence, chapter banner sticky and chip centered, floating math glyphs in headers, quiz reaction emojis

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/components/ui.tsx

@@ -37,6 +37,10 @@ export function ScreenHeader({
     >
       <div className="pointer-events-none absolute -left-12 -top-12 h-40 w-40 rounded-full bg-white/10" />
       <div className="pointer-events-none absolute -right-10 top-10 h-28 w-28 rounded-full bg-white/10" />
+      {/* نمادهای ریاضی که آرام شناورند؛ فقط transform/opacity و بی‌تصویر، پس ارزان‌اند. */}
+      <span aria-hidden className="glyph pointer-events-none absolute left-[42%] top-5 text-3xl font-black text-white/15" style={{ animationDelay: '0s' }}>π</span>
+      <span aria-hidden className="glyph pointer-events-none absolute right-[34%] bottom-3 text-2xl font-black text-white/15" style={{ animationDelay: '-2s' }}>∑</span>
+      <span aria-hidden className="glyph pointer-events-none absolute left-[26%] bottom-2 text-2xl font-black text-white/15" style={{ animationDelay: '-4s' }}>√</span>
       <div className="relative z-10 pt-6">
         {back && (typeof back === 'function' || nav.canGoBack) && (
           <IconButton label="بازگشت" onClick={onBack} className="mb-3 bg-white/15">