Add Reed-Solomon error correction to DCT steganography

- Add reedsolo library for RS error correction (32 symbols = 16 byte correction per 223-byte chunk)
- Protect entire payload (header + data) with RS encoding
- Store 3 copies of length header with majority voting for robustness
- Handle RS chunking overhead (varies based on data size)
- Update capacity calculation to account for RS overhead (24 bytes prefix + variable RS overhead)
- Add RS to dct, web, and api optional dependencies
- Update about.html with v4.1.0 Reed-Solomon feature
- Update module docstring

This fixes DCT decode failures with certain carrier images that have
uniform areas causing unstable DCT coefficients.

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

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

pyproject.toml

@@ -48,6 +48,7 @@ dct = [
     "numpy>=2.0.0",
     "scipy>=1.10.0",
     "jpegio>=0.2.0",
+    "reedsolo>=1.7.0",
 ]
 cli = [
     "click>=8.0.0",
@@ -67,6 +68,7 @@ web = [
     "numpy>=2.0.0",
     "scipy>=1.10.0",
     "jpegio>=0.2.0",
+    "reedsolo>=1.7.0",
 ]
 api = [
     "fastapi>=0.100.0",
@@ -78,6 +80,7 @@ api = [
     "numpy>=2.0.0",
     "scipy>=1.10.0",
     "jpegio>=0.2.0",
+    "reedsolo>=1.7.0",
 ]
 all = [
     "stegasoo[cli,web,api,dct,compression]",