Add audio-sync waveform panel

[RufanMelfor]

Adds a collapsible "Audio Sync Waveform" panel to the calibration
controls, showing overlaid left/right peak-amplitude waveforms for a
window of audio around the playhead. A sync_offset frame error shows
up as a visible shift between the two traces, instead of requiring a
guess-and-check loop against the rendered preview.

• reco-io: calibration_io::extract_audio_pcm_window() extracts a
  windowed audio segment (ffmpeg -ss/-t) instead of always reading
  from the start of the file; extract_audio_pcm() now delegates to it.
• reco-gui: new waveform module downsamples PCM into a smoothed
  peak envelope, normalized per L/R pair so a real mic-level mismatch
  between the two cameras stays visible rather than being independently
  stretched away. A background thread re-extracts it as the playhead
  moves (throttled, polled from the always-running timer tick so it
  keeps updating while paused, not only while playing/seeking).
• Window width (frames around the playhead) and panel height are
  live-adjustable.
• Changing sync_offset forces a recompute so the panel reflects the
  new offset instead of showing a stale envelope.

This is a focused port of the audio-sync waveform panel only - it does
not include the separate "auto-detect sync offset" feature (IMU/audio
cross-correlation), which is unrelated UI/logic and can be its own PR
if wanted.

Test plan

• cargo fmt --all -- --check
• cargo check -p reco-gui --all-targets (full link skipped locally
  to avoid clobbering a running dev build; cargo build was
  confirmed to succeed up to the link step)
• cargo test -p reco-gui -p reco-io — all passing, including 12 new
  waveform::tests unit tests
• cargo clippy -p reco-gui -p reco-io --all-targets -- -D warnings
  — no new findings; failures shown are the same pre-existing
  reco-core dead-code/unsafe-ptr issues already tracked by Clear pre-existing clippy -D warnings failures on Windows #423

[github-actions]

All contributors have signed the CLA. Thank you!
_{Posted by the CLA Assistant Lite bot.}

[RufanMelfor]

I have read the CLA Document and I hereby sign the CLA