colorTheme: Wheel of the Year seasonal themes

Replaced astronomical season detection with the traditional Wheel of the Year sabbat dates. Eight sabbats: Samhain, Yule, Imbolc, Ostara, Beltane, Litha, Lughnasadh, Mabon. Each sabbat triggers a theme transition (e.g. Samhain → dark purple, Yule → deep blue, Beltane → bright green). The theme store tracks isSeasonal state and exposes resetToSeasonal(). Auto-updates at midnight to catch date-based sabbat transitions without requiring a page reload. Expanded seasonal themes from 4 (astronomical seasons) to 8 (full Wheel of the Year coverage with fixed calendar dates).