Add --channel-key flag to generate command

- stegasoo generate --channel-key now outputs a 256-bit hex key
- Also added .env.example template for Web UI configuration

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

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

.gitignore

@@ -54,7 +54,7 @@ htmlcov/
 
 # Environment
 .env
-.env.*
+.env.local
 *.log
 
 # Distribution