Adds a general-purpose SSTV decoder alongside the existing ISS SSTV mode,
supporting USB/LSB/FM modulation on common amateur radio HF/VHF/UHF
frequencies (14.230 MHz USB, 3.845 MHz LSB, etc.) with auto-detection
of modulation from preset frequency table.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
