feat: add dag_engine app — CLI + web frontend for DAG execution (0007e)

Full DAG engine app with CLI subcommands (run, list, status, validate, server)
and React/Mantine web frontend. Uses net/http + embedded Vite build. SQLite
store for run history. Scheduler with cron_ticker for automated execution.
Compatible with existing dagu YAML format.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/dag_engine/frontend/src/main.tsx

@@ -0,0 +1,18 @@
+import "@mantine/core/styles.css";
+import { MantineProvider, createTheme } from "@mantine/core";
+import { createRoot } from "react-dom/client";
+import { BrowserRouter } from "react-router-dom";
+import { App } from "./App";
+
+const theme = createTheme({
+  primaryColor: "blue",
+  fontFamily: "system-ui, -apple-system, sans-serif",
+});
+
+createRoot(document.getElementById("root")!).render(
+  <MantineProvider theme={theme} defaultColorScheme="dark">
+    <BrowserRouter>
+      <App />
+    </BrowserRouter>
+  </MantineProvider>
+);