[codex] add history audio storage and export

[hehehai]

Summary

• add shared history audio storage, export, and playback support across transcription, translation, rewrite, and meeting history
• add history audio settings UI with opt-in storage toggle, storage path selection, export action, and saved-audio stats
• persist remote/MLX/Whisper/Direct Dictation audio into history and fix the session-end cleanup race that dropped staged history audio before import

Why

Issue #54 needs local audio retention and export for saved history items. The original implementation also exposed a race in the remote ASR path where the completed temporary WAV could be staged successfully and then be discarded during session teardown before the history entry imported it.

User impact

• users can opt in to saving history audio and choose where new audio archives are stored
• history detail views can play saved audio when it exists
• export now copies all saved history audio into a chosen folder
• deleting history, clearing all history, and retention cleanup all remove the corresponding audio files too

Validation

• xcodebuild build -scheme Voxt -destination 'platform=macOS' CODE_SIGNING_ALLOWED=NO
• verified remote ASR logging path now stages the completed WAV, preserves it through session end, and imports it into history

Notes

• this PR references Support storing voice data locally in a specified directory #54
• PR is opened as draft because the audio-storage flow had a few rounds of follow-up fixes and debug instrumentation during implementation