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/handlers.go

@@ -42,6 +42,8 @@ type pageData struct {
 	SiteTitle string
 	Channels  []Channel
 	Active    string // slug of active channel, "" for landing
+	Weather   Weather
+	Phase     string // when set, forces day/dawn/dusk/night and disables the clock-driven phase JS
 }
 
 type channelPage struct {
@@ -75,6 +77,7 @@ func (s *Server) base() pageData {
 	return pageData{
 		SiteTitle: s.cfg.SiteTitle,
 		Channels:  channels,
+		Weather:   currentWeather(time.Now()),
 	}
 }
 
@@ -172,6 +175,66 @@ func (s *Server) handleChannel(w http.ResponseWriter, r *http.Request, ch Channe
 	s.render(w, "channel", data)
 }
 
+var (
+	demoVariants    = []string{"rain", "petals", "jacaranda", "motes", "leaves"}
+	demoIntensities = []string{"light", "medium", "heavy"}
+	demoPhases      = []string{"day", "dawn", "dusk", "night"}
+)
+
+type weatherDemoPage struct {
+	pageData
+	Variants    []string
+	Intensities []string
+	Phases      []string
+	Variant     string
+	Intensity   string
+	PhaseSel    string
+}
+
+func pickOr(v string, allowed []string, fallback string) string {
+	for _, a := range allowed {
+		if v == a {
+			return v
+		}
+	}
+	return fallback
+}
+
+func seasonForVariant(v string) string {
+	switch v {
+	case "rain":
+		return "winter"
+	case "petals", "jacaranda":
+		return "spring"
+	case "motes":
+		return "summer"
+	case "leaves":
+		return "autumn"
+	}
+	return ""
+}
+
+func (s *Server) handleWeatherDemo(w http.ResponseWriter, r *http.Request) {
+	q := r.URL.Query()
+	variant := pickOr(q.Get("variant"), demoVariants, "jacaranda")
+	intensity := pickOr(q.Get("intensity"), demoIntensities, "heavy")
+	phase := pickOr(q.Get("phase"), demoPhases, "day")
+
+	base := s.base()
+	base.Weather = Weather{Season: seasonForVariant(variant), Variant: variant, Intensity: intensity}
+	base.Phase = phase
+
+	s.render(w, "weather", weatherDemoPage{
+		pageData:    base,
+		Variants:    demoVariants,
+		Intensities: demoIntensities,
+		Phases:      demoPhases,
+		Variant:     variant,
+		Intensity:   intensity,
+		PhaseSel:    phase,
+	})
+}
+
 func (s *Server) render(w http.ResponseWriter, page string, data any) {
 	tpl, ok := s.tpls[page]
 	if !ok {