Fix WeFax error detection and surface errors in strip UI

rtl_fm subprocess failures (missing tool, no SDR hardware) were silent —
add tool-path check and post-spawn health check in _start_pipeline(),
show errors prominently in the strip status bar (red text + red dot),
and include error detail in scheduler skip events.

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

utils/wefax_scheduler.py

@@ -406,6 +406,7 @@ class WeFaxScheduler:
                 'type': 'schedule_capture_skipped',
                 'broadcast': sb.to_dict(),
                 'reason': 'start_failed',
+                'detail': decoder.last_error or 'unknown error',
             })
 
     def _stop_capture(