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/workspace/messages/message-list.tsx

@@ -166,7 +166,7 @@ export function MessageList({
               results.push(
                 <div
                   key="subtask-count"
-                  className="text-muted-foreground font-norma pt-2 text-sm"
+                  className="text-muted-foreground pt-2 text-sm font-normal"
                 >
                   {t.subtasks.executing(tasks.size)}
                 </div>,

frontend/src/components/workspace/streaming-indicator.tsx

@@ -14,19 +14,19 @@ export function StreamingIndicator({
       <div
         className={cn(
           dotSize,
-          "animate-bouncing rounded-full bg-[#a3a1a1] opacity-100",
+          "animate-bouncing bg-muted-foreground rounded-full opacity-100",
         )}
       />
       <div
         className={cn(
           dotSize,
-          "animate-bouncing rounded-full bg-[#a3a1a1] opacity-100 [animation-delay:0.2s]",
+          "animate-bouncing bg-muted-foreground rounded-full opacity-100 [animation-delay:0.2s]",
         )}
       />
       <div
         className={cn(
           dotSize,
-          "animate-bouncing rounded-full bg-[#a3a1a1] opacity-100 [animation-delay:0.4s]",
+          "animate-bouncing bg-muted-foreground rounded-full opacity-100 [animation-delay:0.4s]",
         )}
       />
     </div>

frontend/src/components/workspace/welcome.tsx

@@ -61,7 +61,9 @@ export function Welcome({
       ) : (
         <div className="text-muted-foreground text-sm">
           {t.welcome.description.includes("\n") ? (
-            <pre className="whitespace-pre">{t.welcome.description}</pre>
+            <pre className="font-sans whitespace-pre">
+              {t.welcome.description}
+            </pre>
           ) : (
             <p>{t.welcome.description}</p>
           )}