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/lib/api/query-client.ts

@@ -0,0 +1,23 @@
+"use client";
+
+import { QueryClient } from "@tanstack/react-query";
+
+export function makeQueryClient() {
+  return new QueryClient({
+    defaultOptions: {
+      queries: {
+        staleTime: 30_000,
+        refetchOnWindowFocus: false,
+        retry: 1,
+      },
+    },
+  });
+}
+
+let browserQueryClient: QueryClient | undefined;
+
+export function getQueryClient() {
+  if (typeof window === "undefined") return makeQueryClient();
+  if (!browserQueryClient) browserQueryClient = makeQueryClient();
+  return browserQueryClient;
+}