feat(monitor/frontend): audio chime infrastructure (load/unlock/play, phase 6 wires the trigger)

Per spec §10.5 — three pure functions sitting on top of the existing
useAudioStore (so React surfaces can subscribe to .unlocked if a
"click anywhere to enable audio" toast is needed later):

  loadChime(file)              fetch / decode arraybuffer into AudioBuffer
  unlockOnFirstGesture()       attach one-shot pointerdown listener that
                               resumes a suspended AudioContext, sets
                               unlocked=true. Idempotent across re-mounts
                               via module-level listenerAttached guard
                               (StrictMode double-mount safe).
  playChime() → boolean        returns false if the audio context isn't
                               ready / buffer not loaded / not unlocked,
                               so callers can fall back to a toast.

Phase 6 calls playChime() on alert match; Phase 5 only wires the
unlockOnFirstGesture() call into the dashboard lifecycle (Task 24)
so the audio is ready when Phase 6 lands.

PROJECTS/advanced/monitor-the-situation-dashboard/frontend/src/lib/audio.ts

@@ -0,0 +1,47 @@
+// ©AngelaMos | 2026
+// audio.ts
+
+import { useAudioStore } from '@/stores/audio'
+
+let listenerAttached = false
+
+function ensureCtx(): AudioContext {
+  let ctx = useAudioStore.getState().ctx
+  if (!ctx) {
+    ctx = new AudioContext()
+    useAudioStore.getState().setCtx(ctx)
+  }
+  return ctx
+}
+
+export async function loadChime(file: File): Promise<void> {
+  const ctx = ensureCtx()
+  const ab = await file.arrayBuffer()
+  const buffer = await ctx.decodeAudioData(ab)
+  useAudioStore.getState().setBuffer(buffer)
+}
+
+export function unlockOnFirstGesture(): void {
+  if (useAudioStore.getState().unlocked) return
+  if (listenerAttached) return
+  listenerAttached = true
+
+  const handler = async (): Promise<void> => {
+    listenerAttached = false
+    const ctx = ensureCtx()
+    if (ctx.state === 'suspended') await ctx.resume()
+    useAudioStore.getState().setUnlocked(true)
+  }
+
+  document.addEventListener('pointerdown', handler, { once: true })
+}
+
+export function playChime(): boolean {
+  const { ctx, buffer, unlocked } = useAudioStore.getState()
+  if (!ctx || !buffer || !unlocked) return false
+  const src = ctx.createBufferSource()
+  src.buffer = buffer
+  src.connect(ctx.destination)
+  src.start(0)
+  return true
+}