Canvas-based seasonal weather effects + demo page

Server picks variant from Lisbon-local date (rain/petals/jacaranda/motes/leaves)
and renders behind content via a canvas particle layer. Each variant has a
hand-drawn silhouette so shapes are recognizable. /weather demo route exposes
variant + intensity + phase pickers, locking the time-of-day phase override.

internal/web/season.go

@@ -0,0 +1,39 @@
+package web
+
+import "time"
+
+// Weather is the seasonal effect rendered behind the page. Tuned for Lisbon:
+// winter is rainy (not snowy), May gets its own jacaranda variant since the
+// city turns purple for a few weeks each year, and September stays in summer
+// because Portugal's dry spell runs all the way through it.
+type Weather struct {
+	Season    string // "winter" | "spring" | "summer" | "autumn"
+	Variant   string // "rain" | "petals" | "jacaranda" | "motes" | "leaves"
+	Intensity string // "light" | "medium" | "heavy"
+}
+
+var lisbon = func() *time.Location {
+	loc, err := time.LoadLocation("Europe/Lisbon")
+	if err != nil {
+		return time.UTC
+	}
+	return loc
+}()
+
+func currentWeather(now time.Time) Weather {
+	t := now.In(lisbon)
+	var season, variant string
+	switch t.Month() {
+	case time.December, time.January, time.February:
+		season, variant = "winter", "rain"
+	case time.March, time.April:
+		season, variant = "spring", "petals"
+	case time.May:
+		season, variant = "spring", "jacaranda"
+	case time.June, time.July, time.August, time.September:
+		season, variant = "summer", "motes"
+	default: // Oct, Nov
+		season, variant = "autumn", "leaves"
+	}
+	return Weather{Season: season, Variant: variant, Intensity: "heavy"}
+}