Redesign UI, add subject catalog & steps

Major refactor of index.html: rename app title and redesign styles (CSS variables, layout, responsive rules and component styles) for a cleaner, step-based UI. Sidebar reworked into guided steps (Asignatura → Grupo y horario → Calendario), added a built-in SUBJECTS_CCH catalog and search/select controls, plus a temporary custom-subject panel. Moved/renamed form fields (programHours, weeklyHours, start/end dates, holidays, vacations), improved group/schedule/unit inputs and lists, and updated report area copy. Kept PDF export libs and added initial JS catalog data and helpers. Overall aims: clearer workflow, better responsive behavior and support for validating program hours and DGIRE comparison.

Author: arantxa-garcri