fix: comprehensive repo audit — 15 issues fixed

test(frontend): Sprint 5 — Vitest setup, 14 component/API tests, Viewer memo

fix: Sprint 3a — updated_at onupdate, RetroMenuBar unique keys

fix(frontend): Sprint 2b — dead code, accessibility, Viewer robustness, ApiError

feat: IIIF-native Sprints 4-5-6 — tiled zoom, exports, cleanup

feat(frontend): Sprint Fix 4 — React Router, accessibility, translations

fix(frontend): Sprint Fix 3 — type safety, memory leaks, error handling

fix(frontend): fix broken navigation, buttons, and layout issues

feat(frontend): Sprint R3 — retro Reader / Manuscript viewer

feat(frontend): Sprint R2 — retro Home / Desktop

feat(frontend): Sprint R1 — retro design system foundation

feat(sprint6-B): validation éditoriale + recherche plein texte

feat(frontend): interface d'administration — Sprint 6 Session A

feat(sprint5): frontend React minimal — visionneuse + 4 couches