Add a status-bar toggle to disable the falling petals

Some people find the drifting ambient petals distracting rather than
cozy, so make the whole PetalFall layer opt-out-able from a 🌸 toggle
sitting just left of the sound toggle in the status bar.

Mirrors the existing sound-mute pattern: a tiny localStorage-backed
pub/sub (effects/petals.ts) drives a PetalsToggle twin of SoundToggle.
PetalFall subscribes to the setting, stops animating and wipes the
canvas when off, and the choice persists across reloads. Works in both
day (petals) and night (stars) modes.

Claude-Session: https://claude.ai/code/session_016Yr6jELuRc7hyzYLccQKZd

web/src/effects/petals.ts

@@ -0,0 +1,36 @@
+// The on/off preference for the ambient falling-petals layer (PetalFall). Some
+// people find drifting petals distracting rather than cozy, so the whole effect
+// is opt-out-able and the choice persists in localStorage so it survives reloads.
+// Mirrors the tiny pub/sub used for the sound mute toggle (../audio/sounds).
+
+const STORAGE_KEY = 'petal.petals'
+
+let enabled = readEnabled()
+const listeners = new Set<(on: boolean) => void>()
+
+function readEnabled(): boolean {
+  try {
+    return localStorage.getItem(STORAGE_KEY) !== 'off'
+  } catch {
+    return true
+  }
+}
+
+export function isPetalsEnabled(): boolean {
+  return enabled
+}
+
+export function setPetalsEnabled(on: boolean): void {
+  enabled = on
+  try {
+    localStorage.setItem(STORAGE_KEY, on ? 'on' : 'off')
+  } catch {
+    /* private mode — choice just won't persist */
+  }
+  listeners.forEach((fn) => fn(on))
+}
+
+export function onPetalsEnabledChange(fn: (on: boolean) => void): () => void {
+  listeners.add(fn)
+  return () => listeners.delete(fn)
+}