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/components/StatusBadge.tsx

@@ -0,0 +1,18 @@
+import { Badge } from "@mantine/core";
+
+const colorMap: Record<string, string> = {
+  success: "green",
+  failed: "red",
+  running: "blue",
+  pending: "gray",
+  cancelled: "yellow",
+  skipped: "dimmed",
+};
+
+export function StatusBadge({ status }: { status: string }) {
+  return (
+    <Badge color={colorMap[status] || "gray"} variant="light" size="sm">
+      {status}
+    </Badge>
+  );
+}