fix: resume AudioContext before playback to fix sounds on Android

[xfalcox]

On Android/mobile, new AudioContext() starts in suspended state per the Web Audio autoplay policy. Without an explicit resume(), no audio is produced — desktop browsers auto-resume, which is why the bug was mobile-only.

Changes

• Shared AudioContext — replaced per-call new AudioContext() with a lazily-initialized module-level singleton (sharedCtx), avoiding browser limits on concurrent contexts (~6)
• ctx.resume() before scheduling — getAudioContext() resumes the context if suspended before returning it, unblocking playback on mobile
• Removed ctx.close() on onended — closing a shared context would break all subsequent sounds; nodes are disposed automatically by the GC after they finish

// Before: new context per sound, no resume, closed on ended
export function playUserJoinedSound() {
  const ctx = new AudioContext(); // suspended on Android
  const now = ctx.currentTime;   // 0, nothing schedules
  // ...
  osc.onended = () => ctx.close();
}

// After: shared context, resumed before use
export async function playUserJoinedSound() {
  const ctx = await getAudioContext(); // resumes if suspended
  const now = ctx.currentTime;
  // ...
}