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/chat.tsx

@@ -459,7 +459,7 @@ export function Chat({
 				    when it disappears and slide the title + chip up. */}
 				<header className="flex h-10 items-center gap-3 px-3">
 					<SidebarToggle />
-					<div className="min-w-0 flex-1 truncate font-medium">{title}</div>
+					<div className="min-w-0 flex-1 truncate font-title text-2xl tracking-tight">{title}</div>
 					<PhaseChip phase={phases.at(-1) ?? "probe"} />
 				</header>