Add RINEX observation file visualization

- RINEX 3.x obs parser in JavaScript (header, SNR extraction,
  multi-line SYS/OBS TYPES, epoch parsing, auto-downsample)
- Per-satellite SNR time series panel (colored by constellation)
- Observation summary panel (version, marker, interval, per-system
  mean SNR and observation type lists)
- Drag & drop .obs/.rnx files alongside .pos files
- Skyplot SNR update from real obs data when PRNs match
- Clear Obs button to remove observation data
- Tested with real RINEX 3.02 data (GPS/Galileo/QZSS/SBAS/BeiDou)

Author: rsasaki0109