feat(gui): add FastAPI + Jinja2 + HTMX scaffold

- FastAPI app with Jinja2 templates and Pico CSS + HTMX from CDN
- Routes: GET / (placeholder page), GET /health (JSON healthcheck)
- systemd unit (no Install section - manual start only)
- TestClient tests for both endpoints

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

systemd/central-gui.service

@@ -0,0 +1,20 @@
+[Unit]
+Description=Central GUI (FastAPI + HTMX)
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+User=central
+Group=central
+WorkingDirectory=/opt/central
+Environment=HOME=/opt/central
+ExecStart=/opt/central/.venv/bin/central-gui
+Restart=on-failure
+RestartSec=5
+NoNewPrivileges=true
+ProtectSystem=full
+ProtectHome=true
+PrivateTmp=true
+StandardOutput=journal
+StandardError=journal