Cap RSA at 3072 bits, add zstd compression for QR codes

- RSA key size capped at 3072 bits (4096 too large for QR codes)
- Added zstd compression for QR code RSA keys (better ratio than zlib)
- New prefix STEGASOO-ZS: for zstd, backward compatible with STEGASOO-Z: (zlib)
- Added zstandard dependency to web/api/compression extras
- Updated all docs, CLI options, and web UI to reflect 3072 max
- Version bump to 4.2.0

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

pyproject.toml

@@ -58,6 +58,7 @@ cli = [
 ]
 compression = [
     "lz4>=4.0.0",
+    "zstandard>=0.22.0",
 ]
 web = [
     "flask>=3.0.0",
@@ -65,6 +66,7 @@ web = [
     "qrcode>=7.3.0",
     "pyzbar>=0.1.9",
     "piexif>=1.1.0",
+    "zstandard>=0.22.0",  # v4.2.0: Better compression for QR keys
     # Include DCT support for web UI
     "numpy>=2.0.0",
     "scipy>=1.10.0",
@@ -77,6 +79,7 @@ api = [
     "python-multipart>=0.0.6",
     "qrcode>=7.30",
     "pyzbar>=0.1.9",
+    "zstandard>=0.22.0",  # v4.2.0: Better compression for QR keys
     # Include DCT support for API
     "numpy>=2.0.0",
     "scipy>=1.10.0",