refactor(frontend): restructure project layout and update API schema

Relocate frontend source code from `next-app/` to `frontend/` to align with the new project structure. This includes removing the old Next.js boilerplate files and establishing a cleaner workspace.

Additionally, updates the OpenAPI specification to include support for the `immich` widget type and its corresponding configuration schema.

- Move frontend files to `frontend/`
- Delete obsolete `next-app/` directory and its configuration
- Add `immich` widget type to `openapi.yaml`
- Update `FrontendPlan.md` with dashboard refactor and UX direction

frontend/next.config.ts

@@ -0,0 +1,19 @@
+import type { NextConfig } from "next";
+
+const nextConfig: NextConfig = {
+  output: "standalone",
+  images: {
+    remotePatterns: [
+      { protocol: "https", hostname: "**" },
+      { protocol: "http", hostname: "**" },
+    ],
+  },
+  async rewrites() {
+    const backend = process.env.NEXT_PUBLIC_API_BASE_URL || "http://localhost:8080";
+    return [
+      { source: "/uploads/:path*", destination: `${backend}/uploads/:path*` },
+    ];
+  },
+};
+
+export default nextConfig;