Improve waterfall error handling and SDR tool path resolution

- Add pre-flight check for I/Q capture binary before spawning process
- Capture stderr from I/Q process for better error diagnostics
- Sync effective span value back to UI when backend adjusts it
- Use get_tool_path('rx_sdr') in Airspy, HackRF, LimeSDR, and SDRPlay
  command builders to support custom install locations

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

utils/sdr/airspy.py

@@ -10,6 +10,8 @@ from __future__ import annotations
 
 from typing import Optional
 
+from utils.dependencies import get_tool_path
+
 from .base import CommandBuilder, SDRCapabilities, SDRDevice, SDRType
 
 
@@ -203,8 +205,9 @@ class AirspyCommandBuilder(CommandBuilder):
         device_str = self._build_device_string(device)
         freq_hz = int(frequency_mhz * 1e6)
 
+        rx_sdr_path = get_tool_path('rx_sdr') or 'rx_sdr'
         cmd = [
-            'rx_sdr',
+            rx_sdr_path,
             '-d', device_str,
             '-f', str(freq_hz),
             '-s', str(sample_rate),