Offline sync: custom vocabulary not applied to transcription

[omi-discord-vector]

Problem

Offline-synced recordings do not use custom vocabulary/dictionary words during transcription, resulting in poor transcription quality.

Reproduction

1. Add custom vocabulary words (Settings → Custom Vocabulary)
2. Record offline (device disconnected/no internet)
3. Sync offline recordings later
4. Check transcript → custom words are not recognized (transcribed incorrectly)

Expected

Offline sync should apply same custom vocabulary as live transcription:

• Use user's dictionary words
• Recognize names, technical terms, jargon
• Match transcription quality of live recordings

Actual

• Live sync: custom vocabulary works ✅
• Offline sync: vocabulary ignored, unreliable transcription ❌

Impact

Users who record offline get poor transcripts:

• Names wrong (e.g., "Aarav" → "Arav" or "Arav")
• Technical terms mangled
• Company/product names incorrect

Technical Notes

Offline sync likely uses different transcription endpoint/pipeline that doesn't include custom vocabulary context.

[beastoin]

Consolidated into #6172 which covers the full sync vs realtime feature gap, including custom vocabulary support.