Avoid mutating audioDataArray in WAV export fallback

[maboa]

Background

The WAV export click handler at index.html:1872-1879 pushes a fallback "whole file" AudioData into the shared audioDataArray when it's empty. That mutates module-level state from a click handler. Currently harmless (checkStrikeThrus early-returns when audioDataArray.length < 2) but it's a latent side effect that will bite when more code reads the array.

Proposed change

Build a local export array instead, leave audioDataArray alone.

const exportData = audioDataArray.length > 0
  ? audioDataArray
  : [new AudioData(player.src, 0, player.duration)];
const wavBlob = await cutAudioApp.cutAudio(exportData);

Acceptance criteria

• Export handler does not push into audioDataArray.
• Whole-file fallback still works when no transcript has been loaded.
• No observable behavior change for users.

Notes

May be subsumed by the export-modal work — but small and safe to land independently.