feat(frontend): add next-race preview page (#25)

Assemble /preview from existing schedule, track-outline, results, and
championship hub endpoints with client-side preview.ts helpers, section
loading/empty states, and Vitest coverage for lib + page render paths.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/components/Nav.tsx

@@ -16,6 +16,9 @@ export function Nav() {
         <Link to="/race-hub" search={{}} activeProps={{ className: 'active' }}>
           Race Hub
         </Link>
+        <Link to="/preview" activeProps={{ className: 'active' }}>
+          Preview
+        </Link>
         <Link to="/championship" activeProps={{ className: 'active' }}>
           Championship
         </Link>