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>

CLAUDE.md

@@ -1,7 +1,7 @@
 # Stegasoo — Claude Code Project Guide
 
 Stegasoo is a secure steganography toolkit with hybrid photo + passphrase + PIN authentication.
-Version 4.2.1 · Python >=3.11 · MIT License
+Version 4.3.0 · Python >=3.11 · MIT License
 
 ## Quick commands
 
@@ -27,6 +27,10 @@ src/stegasoo/          Core library
   models.py              Dataclasses (EncodeResult, DecodeResult, etc.)
   encode.py / decode.py  High-level encode/decode orchestration
   channel.py             Channel key management (v4.0+)
+  audio_steganography.py  LSB audio embedding/extraction (v4.3.0)
+  spread_steganography.py Spread spectrum audio embedding (v4.3.0)
+  audio_utils.py          Audio format detection, validation, transcoding (v4.3.0)
+  debug.py                Structured logging for operations (v4.3.0)
   compression.py         Zstandard / zlib / lz4 payload compression
   cli.py                 Click CLI entry point
   generate.py            Credential generation (passphrase, PIN, RSA keys)