feat: Reflect `thread main_image_url` through DTO and ViewModel with thumbnail fallback

[zigzagdev]

Summary

Carry main_image_url through WorldHeritageDto, the factories, and the ViewModel, and make the exposed thumbnail prefer main_image_url over the legacy images[0]->url.

Changes / 対応内容

• Add ?string $mainImageUrl = null to WorldHeritageDto constructor + getter.
• Update WorldHeritageDetailFactory::build and WorldHeritageSummaryFactory::build to read main_image_url from the input array.
• In WorldHeritageViewModel (and toSummaryArray in WorldHeritageDtoCollection), expose thumbnail_url = main_image_url ?? images[0]?->url ?? null.