Add export modal with choice between entire and edited media

[maboa]

Background

The FILE menu's WAV item currently exports whatever's in audioDataArray with no explicit choice point. If the user has redacted words or enabled gap removal, those edits are silently baked into the export. With no transcript loaded, it silently exports the whole file via a fallback. Either way the user doesn't see what they're getting until the file lands.

Proposed change

Replace the single WAV menu item with an "Export…" entry that opens a modal. The modal exposes:

• A choice of source:
  • Entire file — full source media, ignores redactions and gap-skip settings.
  • Edited — strikethrough redactions and silent gaps removed, as currently configured.
• A short hint under "Edited" explaining what's included/excluded based on the current toggles.
• A download button.

Format selection (WAV / MP3 / video) is out of scope for this issue — see follow-ups. For now the modal produces WAV as today.

Acceptance criteria

• FILE menu has a single "Export…" entry that opens the modal.
• Modal lets the user pick "Entire file" or "Edited".
• Edited mode uses the current audioDataArray (which already respects strikethrough + gap-removal settings).
• Entire mode builds a fresh single-section export ignoring audioDataArray.
• The export flow does not mutate audioDataArray as a side effect.
• Modal copy makes clear what "Edited" includes given the current state.

Out of scope

• New formats (MP3, Opus, video) — separate issues.
• Advanced codec/bitrate controls.