Práctica fecha valid-range guard [2025-01-01 .. today AR-tz]

Remove RETROACTIVIDAD_MAX_DAYS 7-day retro guard (too tight for just-launched app; Puia wants current students to backdate already-done prácticas). Hard floor = 2025-01-01 (provisional const, month unconfirmed, reverts to 7-day rule in 2027). Ceiling = no future: reject fecha > today AR-tz; remove the future-allow branch (client model is retrospective per pm/ research; future-allow was a venus code default, never a client ask; Mars/Pluto reject future). Both server-side (crearPractica+editarPractica) AND picker (DateDmyField yearMin 2025 + maxIso today). Client-confirmed Puia via Elazar 2026-07-06.