Wave-5 reskin batch 3: header title in CMU serif, markdown gets lh-prose

- Session title: font-title (CMU Serif bold) at text-2xl, per the mock.
- MessageResponse's markdown container now carries .lh-prose (Source
  Sans 3 + emphasis tokens) on every markdown surface; labels stay
  JetBrains Mono. Unlayered plain CSS beats the text-sm utility.
- PhaseChip + PhaseDivider sizes now pull from the declared text ladder
  (text-xs) instead of pixel literals.

components/ai-elements/message.tsx

@@ -384,7 +384,9 @@ export const MessageResponse = memo(
     return (
       <Streamdown
         className={cn(
-          "size-full [&>*:first-child]:mt-0 [&>*:last-child]:mb-0",
+          // Source Sans 3 + emphasis-color tokens on every markdown surface
+          // (labels stay JetBrains Mono). Unlayered plain CSS wins over text-sm.
+          "lh-prose size-full [&>*:first-child]:mt-0 [&>*:last-child]:mb-0",
           className
         )}
         mermaid={mermaidOptions}