feat(visual): standardize OFDM spectrum view and restore per-channel grouping from measurement stats

- redesign GetCapture-OFDM with standardized header row, optional Capture Time pill, and top Device Info table
- remove fake sysDescr fallbacks and use N/A-safe device info merge logic
- preserve per-channel spectrum charts and details while modernizing layout/styling
- add combined spectrum overlay graph across all channels/captures
- map captures to measurement_stats by frequency-range keys (first/last segment center freq + segment span)
- use measurement_stats.channel_stats OFDM channel ID for grouping when analyses channel_id is not usable
- remove redundant per-channel MacAddress display (shown once in Device Info)
- use fixed-height Chart.js 2 containers to avoid Postman resize issues
- regenerate visual docs/previews for GetCapture-OFDM - 2026-02-23 00:00:52

Author: mgarcia01752