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>,