fix: Add missing SSTV mode specs for HF decoding (PD90/PD160/PD240/ScottieDX)

VIS detection recognized these modes but ALL_MODES had no decoder specs,
causing silent decode failures on common HF frequencies like 14.230 MHz.
Also emit a user-visible SSE event when an unsupported VIS code is detected.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

utils/sstv/sstv_decoder.py

@@ -490,6 +490,10 @@ class SSTVDecoder:
                             ))
                         else:
                             logger.warning(f"No mode spec for VIS code {vis_code}")
+                            self._emit_progress(DecodeProgress(
+                                status='detecting',
+                                message=f'Detected unknown mode (VIS {vis_code}: {mode_name}) - unsupported',
+                            ))
 
                     # Emit signal level metrics every ~500ms (every 5th 100ms chunk)
                     scope_tone: str | None = None