Phase 3 - UI shell

[SirWellington]

PR #3 implements the Phase 3 UI shell for Praetorian, creating a production-ready Lightroom-inspired React interface with a dark-mode layout (sidebar, main gallery grid, and right details panel). The PR adds the complete frontend scaffold with mock data, virtual scrolling for thumbnails, and details panels with EXIF metadata display—all while the backend remains mockable for future integration.