Add date-driven seasonal ambient background

Details

Add a second full-viewport canvas layer (behind the mesh and content)
that renders a modest, season-appropriate particle effect chosen from
the current date:
  winter — drifting snow with a gentle wind (igloo.inc style)
  spring — falling cherry-blossom petals
  summer — warm light motes drifting upward
  fall   — tumbling autumn leaves
Particle counts scale with viewport area and are kept sparse and
low-opacity so the effect reads as atmosphere without competing with
the UI. Colors adapt to the light/dark theme, the loop pauses when the
tab is hidden, and prefers-reduced-motion disables it entirely. The
canvas is pointer-events:none so it never intercepts clicks. Append
?season=winter|spring|summer|fall to preview a specific season.

Files changed (3)