Add Guardian Angel, Apostles' Creed, Tantum Ergo, Angelus, and Regina
Caeli to the prayers collection. Move standalone Angelus route into the
prayers system with a 301 redirect from the old path. Extract Easter
computation into shared utility ($lib/js/easter.svelte.ts) and use it
for liturgical season awareness: during Eastertide the rosary defaults
to Glorious mysteries and swaps Salve Regina for Regina Caeli; during
Lent it defaults to Sorrowful mysteries. Seasonal badges shown on both
the mystery selector and prayer sections.
