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>

app/api/gallery-unpack/route.ts

@@ -4,8 +4,8 @@ export const runtime = "nodejs";
 
 // Cap a bit above pack's MAX_DOC_BYTES — ciphertext adds the 16-byte GCM tag
 // and the 17-byte header; some slack accommodates near-cap docs without
-// rejecting them at unpack time.
-const MAX_FILE_BYTES = 6_000_000;
+// rejecting them at unpack time. Bumped to fit pre-baked beat audio.
+const MAX_FILE_BYTES = 13_000_000;
 
 // Decrypt a `.infiplot` share file back to its doc JSON string. Returns the
 // plaintext as a JSON field (not raw bytes) so the client can chain it through