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>

README.md

@@ -1,6 +1,6 @@
 # Stegasoo
 
-A secure steganography system for hiding encrypted messages in images using hybrid authentication.
+A secure steganography system for hiding encrypted messages in images and audio using hybrid authentication.
 
 [![Tests](https://github.com/adlee-was-taken/stegasoo/actions/workflows/test.yml/badge.svg)](https://github.com/adlee-was-taken/stegasoo/actions/workflows/test.yml)
 [![Lint](https://github.com/adlee-was-taken/stegasoo/actions/workflows/lint.yml/badge.svg)](https://github.com/adlee-was-taken/stegasoo/actions/workflows/lint.yml)
@@ -17,15 +17,25 @@ A secure steganography system for hiding encrypted messages in images using hybr
 - **Multiple interfaces**: CLI, Web UI, REST API
 - **File embedding**: Hide any file type (PDF, ZIP, documents)
 - **DCT steganography**: JPEG-resilient embedding for social media
+- **Audio steganography**: Hide messages in WAV, FLAC, MP3, OGG, AAC, M4A files (LSB and Spread Spectrum modes)
 - **Channel keys**: Private group communication channels
 
 ## Embedding Modes
 
+### Image Modes
+
 | Mode | Capacity (1080p) | JPEG Resilient | Best For |
 |------|------------------|----------------|----------|
 | **DCT** (default) | ~150 KB | Yes | Social media, messaging apps |
 | **LSB** | ~750 KB | No | Email, direct file transfer |
 
+### Audio Modes
+
+| Mode | Capacity (5 min WAV) | Noise Resistant | Best For |
+|------|---------------------|-----------------|----------|
+| **LSB** | ~1.3 MB | No | Direct file transfer |
+| **Spread Spectrum** | ~160 KB | Yes | Shared files, light processing |
+
 ## Web UI
 
 | Home | Encode | Decode | Generate |