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>

tests/test_gui_scaffold.py

@@ -0,0 +1,42 @@
+"""Tests for GUI scaffold."""
+
+from fastapi.testclient import TestClient
+
+from central.gui import app
+
+
+client = TestClient(app)
+
+
+class TestHealthEndpoint:
+    """Tests for /health endpoint."""
+
+    def test_health_returns_200(self):
+        """Health endpoint returns 200 OK."""
+        response = client.get("/health")
+        assert response.status_code == 200
+
+    def test_health_returns_status_ok(self):
+        """Health endpoint returns status ok JSON."""
+        response = client.get("/health")
+        assert response.json() == {"status": "ok"}
+
+
+class TestIndexEndpoint:
+    """Tests for / endpoint."""
+
+    def test_index_returns_200(self):
+        """Index endpoint returns 200 OK."""
+        response = client.get("/")
+        assert response.status_code == 200
+
+    def test_index_returns_html(self):
+        """Index endpoint returns HTML content."""
+        response = client.get("/")
+        assert "text/html" in response.headers["content-type"]
+
+    def test_index_contains_placeholder(self):
+        """Index page contains the placeholder text."""
+        response = client.get("/")
+        assert "Central" in response.text
+        assert "coming soon" in response.text.lower()