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>

rpi/setup.sh

@@ -182,6 +182,7 @@ WorkingDirectory=$INSTALL_DIR/frontends/web
 Environment="PATH=$INSTALL_DIR/venv/bin:/usr/bin"
 Environment="STEGASOO_AUTH_ENABLED=true"
 Environment="STEGASOO_HTTPS_ENABLED=false"
+Environment="STEGASOO_PORT=5000"
 ExecStart=$INSTALL_DIR/venv/bin/python app.py
 Restart=on-failure
 RestartSec=5
@@ -217,8 +218,28 @@ echo "  journalctl -u stegasoo -f"
 echo ""
 echo -e "${GREEN}Access Web UI:${NC}"
 PI_IP=$(hostname -I | awk '{print $1}')
-echo "  http://$PI_IP:5000"
+echo "  http://$PI_IP:5000  (default port, configurable via STEGASOO_PORT)"
 echo ""
 echo -e "${YELLOW}Note: On first access, you'll be prompted to create an admin account.${NC}"
 echo ""
+echo -e "${GREEN}Enable HTTPS:${NC}"
+echo "  sudo nano /etc/systemd/system/stegasoo.service"
+echo ""
+echo "  Change: Environment=\"STEGASOO_HTTPS_ENABLED=false\""
+echo "  To:     Environment=\"STEGASOO_HTTPS_ENABLED=true\""
+echo ""
+echo "  Save (Ctrl+O, Enter, Ctrl+X), then:"
+echo "    sudo systemctl daemon-reload"
+echo "    sudo systemctl restart stegasoo"
+echo ""
+echo -e "${GREEN}Private Channel Key (optional):${NC}"
+echo "  Generate a key:"
+echo "    source $INSTALL_DIR/venv/bin/activate"
+echo "    stegasoo generate --channel-key"
+echo ""
+echo "  Add to the service file (same nano command above):"
+echo "    Environment=\"STEGASOO_CHANNEL_KEY=XXXX-XXXX-XXXX-XXXX-XXXX-XXXX-XXXX-XXXX\""
+echo ""
+echo "  This ensures only users with the same key can decode your images."
+echo ""
 echo -e "To start now: ${YELLOW}sudo systemctl start stegasoo${NC}"