Typography round 3 (owner): session titles at the prose size

Titles were 14px (text-base); the user read them as still-small next to
the prose they open into. They now pull --lh-prose-size (16.5px) — the
same token lh-prose/lh-input render at, so the picker's titles and the
session's prose share one standard Source Sans 3 size (text-(length:…) —
the v4 var shorthand alone doesn't type-resolve). Meta stays 11px mono.
Live: title 16.5px vs meta 11px, row 54px. Gates: typecheck, lint, 469/469.

components/sidebar.tsx

@@ -138,7 +138,7 @@ export function SessionRow({
 				    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 text-base">{chat.title}</span>
+				<span className="block truncate font-prose text-(length:--lh-prose-size)">{chat.title}</span>
 				<span className="block text-xs text-muted-foreground">{sessionMeta(chat)}</span>
 			</span>
 		</button>