feat: integrate Pi backend and Pi Web

pi-web/app/api/default-cwd/route.ts

@@ -0,0 +1,19 @@
+import { NextResponse } from "next/server";
+import { mkdirSync } from "fs";
+import { homedir } from "os";
+import { join } from "path";
+import { allowFileRoot } from "@/lib/file-access";
+
+// POST /api/default-cwd
+// Creates ~/pi-cwd-<YYYYMMDD> if it doesn't exist and returns the path.
+export async function POST() {
+  try {
+    const date = new Date().toISOString().slice(0, 10).replace(/-/g, "");
+    const dir = join(homedir(), `pi-cwd-${date}`);
+    mkdirSync(dir, { recursive: true });
+    allowFileRoot(dir);
+    return NextResponse.json({ cwd: dir });
+  } catch (error) {
+    return NextResponse.json({ error: String(error) }, { status: 500 });
+  }
+}