feat(mentions): enlarge card thumbnails, brand scrollbar, skeleton + prefetch previews

perf(frontend): difiere el card index del bundle inicial (Capa 5)

refactor(frontend): extrae useMentions y divide RulesContent (Capa 4)

feat(frontend): accesibilidad WCAG 2.1 AA (Capa 1)

feat(frontend): light/dark theme toggle con tokens de marca

feat(frontend): agrega slate blue como secundario para un 60-30-10 real

feat(frontend): dual-trigger mention picker with card thumbnails

feat(ui): redesign chat page with ChatGPT-style layout and visual polish

feat(ui): add keyword badges with skewed style in answer display

feat(ui): add @mention dropdown for TCG keywords in query input

feat: add components, pages, and tests for frontend MVP