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/types.ts

@@ -12,6 +12,7 @@ export interface Meeting {
   country_name: string
   country_code: string
   country_flag: string
+  circuit_key?: number
   circuit_short_name: string
   date_start: string
   date_end: string
@@ -23,6 +24,7 @@ export interface Session {
   session_key: number
   session_name: string
   session_type: string
+  circuit_key?: number
   meeting_key: number
   date_start: string
   date_end: string