Add per-channel hybrid audio spread spectrum and env feature toggles

Spread spectrum v2: independent per-channel embedding with round-robin
bit distribution, preserving spatial stereo/surround mix. Adaptive chip
tiers (256/512/1024) trade capacity for lossy codec robustness. LFE
channel skipped for 5.1+ layouts. v2 header (20B) with backward-
compatible v0 decode fallback.

Environment toggles (STEGASOO_AUDIO, STEGASOO_VIDEO) gate audio/video
features for minimal builds (e.g. Raspberry Pi image-only). Values:
auto (default, detect deps), 1/true (force on), 0/false (force off).

Web UI fixes: accordion defaults to step 1 on load, chevron arrow
styling, required attribute toggling for audio carrier type switch,
"Images & Mode" renamed to "Reference, Carrier, Mode".

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

RELEASE_NOTES.md

@@ -1,3 +1,45 @@
+# v4.3.0 — Audio Steganography
+
+**Release Date:** 2026-02-27
+
+## Highlights
+
+Stegasoo can now hide messages in audio files! This release adds full audio steganography support with two embedding modes:
+
+- **LSB (Least Significant Bit)**: Embeds data directly in audio sample LSBs. High capacity, best for direct file transfers.
+- **Spread Spectrum**: Spreads data across audio frequencies using pseudo-random sequences. Lower capacity but more resistant to noise and light processing.
+
+## What's New
+
+### Audio Steganography
+- Support for WAV, FLAC, MP3, OGG, AAC, and M4A input formats
+- Automatic transcoding to WAV (16-bit PCM) for embedding
+- Same security model: reference photo + passphrase + PIN/RSA + channel key
+- Full CLI, REST API, and Web UI support
+
+### Unified Web UI
+- Encode and Decode pages now feature a "Carrier Type" selector
+- Switch between Image and Audio modes without leaving the page
+- Audio capacity display shows LSB and Spread Spectrum capacities
+- Audio preview player on encode result page
+
+### New Modules
+- `audio_steganography.py` — LSB audio embedding/extraction
+- `spread_steganography.py` — Spread spectrum embedding/extraction
+- `audio_utils.py` — Audio format detection, validation, transcoding
+- `debug.py` — Structured logging for all operations
+
+## Upgrade Notes
+
+Audio steganography requires `numpy` and `soundfile` packages. Install with:
+```bash
+pip install stegasoo[audio]
+```
+
+For full audio format support (MP3, AAC, etc.), install FFmpeg on your system.
+
+---
+
 ## Stegasoo v4.2.1
 
 ### API Security