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>

frontends/web/.env.example

@@ -0,0 +1,15 @@
+# Stegasoo Web UI Configuration
+# Copy this file to .env and customize
+
+# Authentication (v4.0.2+)
+STEGASOO_AUTH_ENABLED=true
+STEGASOO_HTTPS_ENABLED=false
+STEGASOO_HOSTNAME=localhost
+
+# Channel Key (256-bit hex for private channel isolation)
+# Generate with: python -c "import secrets; print(secrets.token_hex(32))"
+# Leave empty for public mode
+STEGASOO_CHANNEL_KEY=
+
+# Flask settings
+FLASK_ENV=production