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/styles/globals.css

@@ -273,7 +273,7 @@
   --accent: oklch(0.32 0.0036 106.64);
   --accent-foreground: oklch(0.985 0 0);
   --destructive: oklch(0.704 0.191 22.216);
-  --border: oklch(1 0.191 22.216 / 10%);
+  --border: oklch(1 0 0 / 10%);
   --input: oklch(1 0 0 / 15%);
   --ring: transparent;
   --chart-1: oklch(0.488 0.243 264.376);