feat(midsummer): WebAudio Sma grodorna jingle, plays once on first gesture

frontend/src/components/PlayerDashboardFullPage.tsx

@@ -1,5 +1,6 @@
 import React, { useEffect, useState } from 'react';
 import { useLiveData } from '../hooks/useLiveData';
+import { useMidsummerSound } from '../hooks/useMidsummerSound';
 import { PlayerDashboardContent } from './windows/PlayerDashboardWindow';
 import { MidsummerBanner } from './midsummer/MidsummerBanner';
 
@@ -15,6 +16,7 @@ import { MidsummerBanner } from './midsummer/MidsummerBanner';
  */
 export const PlayerDashboardFullPage: React.FC = () => {
   const data = useLiveData();
+  useMidsummerSound();
   const [version, setVersion] = useState('');
 
   // Set tab title.

frontend/src/components/map/MapLayout.tsx

@@ -7,6 +7,7 @@ import { WindowRenderer } from '../windows/WindowRenderer';
 import { RareNotification } from '../effects/RareNotification';
 import { DeathNotification } from '../effects/DeathNotification';
 import { usePlayerColors } from '../../hooks/usePlayerColors';
+import { useMidsummerSound } from '../../hooks/useMidsummerSound';
 import { MidsummerBanner } from '../midsummer/MidsummerBanner';
 import type { DashboardState } from '../../hooks/useLiveData';
 
@@ -16,6 +17,7 @@ interface Props {
 
 export const MapLayout: React.FC<Props> = ({ data }) => {
   const getColor = usePlayerColors();
+  useMidsummerSound();
   const [showHeatmap, setShowHeatmap] = useState(false);
   const [showPortals, setShowPortals] = useState(false);
   const [selectedPlayer, setSelectedPlayer] = useState<string | null>(null);

frontend/src/hooks/useMidsummerSound.ts

@@ -1,2 +1,64 @@
-// Stub — replaced with the full WebAudio synth in Task 8.
-export function playSmaGrodorna(): void {}
+import { useEffect } from 'react';
+import { useMidsummer } from './useMidsummer';
+
+const JINGLE_FLAG = 'mo-midsummer-jingle';
+
+let ctx: AudioContext | null = null;
+
+// Public-domain "Små grodorna" opening phrase (approximation), as
+// [frequencyHz, durationSeconds]. Cheerful major-key triangle tones.
+const MELODY: [number, number][] = [
+  [392, 0.22], [392, 0.22], [392, 0.22], [440, 0.22], [494, 0.42],
+  [494, 0.22], [440, 0.22], [494, 0.22], [523, 0.22], [587, 0.5],
+];
+
+/** Play the jingle once. Safe to call from any user gesture. No-op if
+ *  WebAudio is unavailable. Reuses a single AudioContext (no leak). */
+export function playSmaGrodorna(): void {
+  try {
+    const AC = window.AudioContext || (window as unknown as { webkitAudioContext: typeof AudioContext }).webkitAudioContext;
+    if (!AC) return;
+    if (!ctx) ctx = new AC();
+    if (ctx.state === 'suspended') void ctx.resume();
+    let t = ctx.currentTime + 0.05;
+    for (const [freq, dur] of MELODY) {
+      const osc = ctx.createOscillator();
+      const gain = ctx.createGain();
+      osc.type = 'triangle';
+      osc.frequency.value = freq;
+      gain.gain.setValueAtTime(0.0001, t);
+      gain.gain.exponentialRampToValueAtTime(0.18, t + 0.02);
+      gain.gain.exponentialRampToValueAtTime(0.0001, t + dur);
+      osc.connect(gain).connect(ctx.destination);
+      osc.start(t);
+      osc.stop(t + dur);
+      t += dur;
+    }
+  } catch {
+    /* audio not available — ignore */
+  }
+}
+
+/** Plays the jingle once per session, on the first user gesture, when the
+ *  theme and sound are both on. Browsers block audio before a gesture, so
+ *  we wait for the first pointerdown/keydown. */
+export function useMidsummerSound(): void {
+  const { enabled, soundOn } = useMidsummer();
+  useEffect(() => {
+    if (!enabled || !soundOn) return;
+    if (sessionStorage.getItem(JINGLE_FLAG)) return;
+
+    const fire = () => {
+      sessionStorage.setItem(JINGLE_FLAG, '1');
+      playSmaGrodorna();
+      cleanup();
+    };
+    const cleanup = () => {
+      window.removeEventListener('pointerdown', fire);
+      window.removeEventListener('keydown', fire);
+    };
+    window.addEventListener('pointerdown', fire);
+    window.addEventListener('keydown', fire);
+    return cleanup;
+  }, [enabled, soundOn]);
+}