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/package.json

@@ -0,0 +1,28 @@
+{
+  "name": "dag-engine-frontend",
+  "private": true,
+  "version": "1.0.0",
+  "type": "module",
+  "scripts": {
+    "dev": "vite",
+    "build": "tsc -b && vite build",
+    "preview": "vite preview"
+  },
+  "dependencies": {
+    "@mantine/core": "^9.0.2",
+    "@mantine/hooks": "^9.0.2",
+    "@tabler/icons-react": "^3.31.0",
+    "react": "^19.1.0",
+    "react-dom": "^19.1.0",
+    "react-router-dom": "^7.6.1"
+  },
+  "devDependencies": {
+    "@types/react": "^19.1.6",
+    "@types/react-dom": "^19.1.6",
+    "@vitejs/plugin-react": "^4.5.2",
+    "postcss": "^8.5.4",
+    "postcss-preset-mantine": "^1.17.0",
+    "typescript": "~5.8.3",
+    "vite": "^6.3.5"
+  }
+}