feat(fs-sync, mac): add macOS AAC-in-MP4 fallback via afconvert; extract crates/afconvert

Summary

Some MP4 files recorded on macOS (ISOMP4 container with H.264 video + AAC-LC audio) fail to decode via Symphonia (rodio backend) due to AAC validation in their MP4 demuxer/decoder. This PR adds a macOS-only fallback path in plugins/fs-sync/src/audio.rs:

• When rodio::Decoder::try_from(file) fails on macOS:
  • Use a new reusable crate crates/afconvert to call the system afconvert tool to extract audio to a temporary 16-bit LE WAV.
  • Re-open that WAV with rodio and continue the existing resample → mono mixdown → Vorbis encode flow.
  • Clean up the temporary WAV file in all paths.

Additionally:

• New crates/afconvert crate encapsulates the afconvert invocation and error handling for reuse elsewhere.
• Wire hypr-afconvert as a workspace dependency and add it to plugins/fs-sync.

Scope of change is tightly limited to the import path; no other audio capture or STT flows are modified. On non-macOS platforms, behavior is unchanged. On macOS, the fallback only triggers if rodio decoding fails.

Changes of interest:

• crates/afconvert: tiny crate exposing to_wav(&Path) -> Result<PathBuf, Error>.
• fs-sync/audio.rs: refactor to a helper import_audio_from_decoder and add macOS fallback path using hypr_afconvert::to_wav.
• fs-sync/error.rs: add AudioProcessingError::AfconvertFailed(String) for clear surfacing of conversion errors.

Review & Testing Checklist for Human (3 items)

• macOS manual verification with a known-problematic MP4 (H.264 video, AAC-LC audio as described). Import the file into a session and confirm:
  • The fallback path is used (rodio initially fails, no unhandled errors shown).
  • audio.ogg is created and playable; transcription paths still work.
  • No stray temp files remain in /tmp (or std::env::temp_dir()).
• Regression check on macOS and Linux with common formats (wav, mp3, ogg, flac, m4a): ensure imports succeed and no unexpected fallback is taken when rodio succeeds normally.
• Code review for platform gating and dependency wiring:
  • hypr-afconvert usage only inside #[cfg(target_os = "macos")] block for the fallback call.
  • Workspace dependency alias is correct (hypr-afconvert in Cargo.toml, referenced as hypr_afconvert in code).
  • Temporary file cleanup executes on both success and error paths.

Recommended test plan:

1. On macOS:
   • Import an MP4 from macOS screen recording (ISOMP4 with AAC-LC) that previously failed in-app. Confirm the file imports and audio.ogg is generated.
   • Import an audio-only M4A and MP3 to ensure we don’t regress the happy path.
2. On Linux:
   • Import the sample files in crates/data/src/english_1/* to ensure nothing regresses.
3. If available, quickly run the fs-sync tests: cargo test -p tauri-plugin-fs-sync (already passing locally).

Notes

• The afconvert tool is part of macOS and the fallback should only be hit when rodio fails. If afconvert is unavailable or fails, a clear AfconvertFailed error is returned.
• No changes to the main audio processing pipeline besides the decode fallback; encoding, sample rate (16k), mono mixdown, and ogg handling remain identical.
• Link to Devin run (context and discussion): https://app.devin.ai/sessions/65f0601bcc694a579b7ce584a47229e7
  Requested by: @yujonglee