Add STEGASOO_PORT env var, improve RPi setup output, channel key accordion

- Add STEGASOO_PORT environment variable support (default: 5000)
- Update .env.example with port and fix channel key format docs
- Move channel key generation to collapsible accordion in Generate page
- Improve RPi setup.sh output with HTTPS and channel key instructions
- Add rpi/BUILD_IMAGE.md workflow documentation

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

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

frontends/web/app.py

@@ -1405,9 +1405,10 @@ if __name__ == "__main__":
     else:
         print("Authentication disabled")
 
+    port = int(os.environ.get("STEGASOO_PORT", "5000"))
     app.run(
         host="0.0.0.0",
-        port=5000,
+        port=port,
         debug=False,
         ssl_context=ssl_context,
     )