feat(web): embed beat audio into gallery and infiplot exports

Walk every speaking beat at export time, reuse current scene's beatAudioMap,
and synth the rest via BYO TTS or /api/beat-audio with concurrency 4. Show a
progress toast on the play page while collecting.

Gallery export keeps audio in a sidecar localStorage key so the first paint
is not blocked by JSON.parse-ing several MB of base64; the gallery lazy-loads
it after the first scene image, then plays per-beat audio with a mute toggle
persisted to localStorage. .infiplot share files embed audioByBeatId in the
doc itself (v2); on import the data URIs survive scene swaps and feed back
into the per-beat audio map so replayers hear the original voices for free.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

lib/analytics.ts

@@ -53,7 +53,7 @@ type AnalyticsEventData = {
   tts_toggle: { muted: boolean };
   fullscreen_toggle: { on: boolean };
   play_heartbeat: never;
-  gallery_export: { scene_count: number };
+  gallery_export: { scene_count: number; audio_count: number };
 };
 
 export type AnalyticsEvent = keyof AnalyticsEventData;