Make seasonal effects more present; add winter wind…

Details

Strengthen all four seasons so they read clearly instead of as faint
specks:
- winter: wind-blown snow with steady wind + shaped gusts and motion
  streaks, so the wind is actually felt (igloo.inc style)
- spring: larger cherry-blossom petals, more of them
- fall: bigger, proper leaf-shaped particles with veins in autumn
  browns/rust instead of small ellipses
- summer: replace the ambiguous dots with a sun — radial glow, slow
  god-rays and a warm wash so the whole site reads sunny
Still theme-aware, pointer-events:none, pauses when hidden, and
disabled under prefers-reduced-motion.

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