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/mocks/browser.ts

@@ -0,0 +1,16 @@
+"use client";
+
+import { handlers } from "./handlers";
+
+let installed = false;
+
+export function installMocks() {
+  if (installed || typeof window === "undefined") return;
+
+  import("msw/browser").then(({ setupWorker }) => {
+    const worker = setupWorker(...handlers);
+    worker.start({ onUnhandledRequest: "bypass" });
+    installed = true;
+    console.log("[MSW] Mock Service Worker installed");
+  });
+}