Rebuild weather effects on WebGL2 with new variants and a live demo page

The background weather is now GPU-rendered: one instanced-quad draw call
over a baked sprite atlas plus a fullscreen sky shader (fog, Saharan
haze, aurora, sun rays, storm gloom and lightning flash). The old
Canvas2D renderer stays as weather-2d.js and kicks in automatically
when WebGL2 is missing; weather.js is now a thin controller that owns
the toggle, prefs and the PeteWeather API.

Effect upgrades: shared wind with gust pulses leans the whole scene
together, rain gets depth, splash pops and velocity-aligned streaks,
storms grow procedural branched lightning bolts, snow mixes soft motes
with spinning six-arm crystals, clouds drift in two parallax layers,
clear nights get a moon with maria, twinkling stars and the occasional
shooting star, blossoms and leaves tumble with a faked 3D flip.

New variants: haze (Saharan calima), wind (autumn gusts with streak
lines), hail (bouncing stones with drizzle) and aurora. The /weather
demo page switches variant, intensity and phase in place without a
reload and shows the active renderer plus an FPS meter.

internal/web/handlers.go

@@ -401,7 +401,7 @@ func (s *Server) handleBookmarks(w http.ResponseWriter, r *http.Request) {
 }
 
 var (
-	demoVariants    = []string{"rain", "petals", "jacaranda", "motes", "leaves", "clear", "clouds", "snow", "fog", "storm"}
+	demoVariants    = []string{"clear", "clouds", "rain", "storm", "hail", "snow", "fog", "haze", "wind", "aurora", "petals", "jacaranda", "motes", "leaves"}
 	demoIntensities = []string{"light", "medium", "heavy"}
 	demoPhases      = []string{"day", "dawn", "dusk", "night"}
 )
@@ -431,10 +431,12 @@ func seasonForVariant(v string) string {
 		return "winter"
 	case "petals", "jacaranda":
 		return "spring"
-	case "motes":
+	case "motes", "haze":
 		return "summer"
-	case "leaves":
+	case "leaves", "wind":
 		return "autumn"
+	case "hail":
+		return "winter"
 	}
 	return ""
 }