feat(effects): extinguish fire with water/rain and ambient SFX volume

Water and rain erase fire under a tight brush hit radius; add looping fire/rain ambients and an Effects sound slider that also scales one-shot effect SFX.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/renderer/control/rainAmbientSfx.ts

@@ -0,0 +1,50 @@
+/** Фоновый звук дождя при наличии дождя на сцене (`public/rain-ambient.mp3`). */
+
+import { getEffectsSfxGain } from './effectsSfxGain';
+
+const RAIN_AMBIENT_BASE_VOLUME = 0.7;
+
+export function rainAmbientSoundUrl(): string {
+  return new URL('rain-ambient.mp3', window.location.href).href;
+}
+
+let ambientEl: HTMLAudioElement | null = null;
+
+function ensureAmbientEl(): HTMLAudioElement {
+  if (ambientEl) return ambientEl;
+  const el = new Audio(rainAmbientSoundUrl());
+  el.loop = true;
+  el.preload = 'auto';
+  ambientEl = el;
+  return el;
+}
+
+function applyVolume(el: HTMLAudioElement): void {
+  el.volume = Math.max(0, Math.min(1, RAIN_AMBIENT_BASE_VOLUME * getEffectsSfxGain()));
+}
+
+/** Включить/выключить loop дождя. */
+export function setRainAmbientActive(active: boolean): void {
+  try {
+    const el = ensureAmbientEl();
+    applyVolume(el);
+    if (active) {
+      if (el.paused) void el.play().catch(() => undefined);
+    } else if (!el.paused) {
+      el.pause();
+      el.currentTime = 0;
+    }
+  } catch {
+    /* ignore */
+  }
+}
+
+/** Обновить громкость, если ambient уже играет. */
+export function syncRainAmbientVolume(): void {
+  if (!ambientEl) return;
+  try {
+    applyVolume(ambientEl);
+  } catch {
+    /* ignore */
+  }
+}