fix(frontend): UI polish - fix CSS typo, dark mode border, and hardcoded colors (#1942)

- Fix `font-norma` typo to `font-normal` in message-list subtask count
- Fix dark mode `--border` using reddish hue (22.216) instead of neutral
- Replace hardcoded `rgb(184,184,192)` in hero with `text-muted-foreground`
- Replace hardcoded `bg-[#a3a1a1]` in streaming indicator with `bg-muted-foreground`
- Add missing `font-sans` to welcome description `<pre>` for consistency
- Make case-study-section padding responsive (`px-4 md:px-20`)

Closes #1940

frontend/src/components/landing/hero.tsx

@@ -69,10 +69,7 @@ export function Hero({ className }: { className?: string }) {
             </div>
           </a>
         )}
-        <p
-          className="mt-8 scale-105 text-center text-2xl text-shadow-sm"
-          style={{ color: "rgb(184,184,192)" }}
-        >
+        <p className="text-muted-foreground mt-8 scale-105 text-center text-2xl text-shadow-sm">
           An open-source SuperAgent harness that researches, codes, and creates.
           With
           <br />

frontend/src/components/landing/sections/case-study-section.tsx

@@ -51,7 +51,7 @@ export function CaseStudySection({ className }: { className?: string }) {
       title="Case Studies"
       subtitle="See how DeerFlow is used in the wild"
     >
-      <div className="container-md mt-8 grid grid-cols-1 gap-4 px-20 md:grid-cols-2 lg:grid-cols-3">
+      <div className="container-md mt-8 grid grid-cols-1 gap-4 px-4 md:grid-cols-2 md:px-20 lg:grid-cols-3">
         {caseStudies.map((caseStudy) => (
           <Link
             key={caseStudy.title}