Fix audio dependency to use ffmpeg instead of sox

The Listening Post actually uses ffmpeg for audio encoding, not sox.
Updated all documentation, setup scripts, and code to reflect this:

- Removed unused find_sox() function from listening_post.py
- Simplified tools endpoint to only check for ffmpeg
- Updated CHANGELOG, README, HARDWARE.md, Dockerfile
- Fixed setup.sh to check for ffmpeg
- Updated frontend warnings to mention ffmpeg

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Dockerfile

@@ -20,8 +20,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     # Pager decoder
     multimon-ng \
     # Audio tools for Listening Post
-    sox \
-    libsox-fmt-all \
+    ffmpeg \
     # WiFi tools (aircrack-ng suite)
     aircrack-ng \
     iw \