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>

src/central/gui/routes.py

@@ -0,0 +1,23 @@
+"""Route handlers for Central GUI."""
+
+from fastapi import APIRouter, Request
+from fastapi.responses import HTMLResponse
+
+router = APIRouter()
+
+
+@router.get("/health")
+async def health() -> dict:
+    """Health check endpoint."""
+    return {"status": "ok"}
+
+
+@router.get("/", response_class=HTMLResponse)
+async def index(request: Request) -> HTMLResponse:
+    """Render the index page."""
+    from central.gui import templates
+
+    return templates.TemplateResponse(
+        request=request,
+        name="index.html",
+    )