feat(visual): standardize OFDM modulation profile header and add combined frequency overlay

- add Device Info block with standardized labels and N/A-safe sysDescr handling
- add Capture Time pill next to the title using formatted pnm_header.capture_time
- simplify per-channel headers to centered Channel labels
- use whole-number MHz labels for per-channel frequency axes
- add combined bottom chart with all channel/profile lines aligned by frequency (linear MHz axis)
- preserve existing per-channel chart behavior and profile plotting
- regenerate visual docs/previews for GetCapture-ModulationProfile - 2026-02-22 22:56:09

Author: mgarcia01752