Typography round 1 (owner): the three-font rule enforced + Source Sans session titles

- The hidden fourth font: Tailwind's font-mono/font-sans utilities fall to
  the SYSTEM stacks (SFMono/Menlo — a font the app never chose), and
  streamdown's code-block chrome, mermaid-error, and the demo-card meta
  all use them — so every teaching-prose code block rendered in the
  system mono. Fixed at the token level: @theme now aliases --font-mono →
  JetBrains Mono and --font-sans → Source Sans 3, so every utility use
  resolves to the app's three fonts (chrome=JetBrains, prose=Source Sans,
  display=CMU bold).
- Session titles: Source Sans 3 (the human-words voice — the same font
  the words were typed in; mono = machine data stays on the meta line).
  One rule, finally applied to the picker too. CMU bold titles are the
  queued experiment if this reads wrong live.

Gates: typecheck, lint, 469/469. Live: title "Source Sans 3" 12.5px,
meta "JetBrains Mono" 11px; code blocks now JetBrains.

components/sidebar.tsx

@@ -133,7 +133,12 @@ export function SessionRow({
 		>
 			<SessionStatus busy={chat.busy} />
 			<span className="min-w-0 flex-1">
-				<span className="block truncate">{chat.title}</span>
+				{/* Titles are the learner's/teacher's WORDS — Source Sans 3, the
+				    same voice the words were typed in (every input is lh-prose);
+				    the meta line below stays JetBrains Mono (machine data). One
+				    rule, finally applied to the picker too (owner, typography
+				    round): mono = chrome, Source Sans = human words. */}
+				<span className="block truncate font-prose">{chat.title}</span>
 				<span className="block text-xs text-muted-foreground">{sessionMeta(chat)}</span>
 			</span>
 		</button>