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/vitest.config.ts

@@ -0,0 +1,13 @@
+import { defineConfig } from "vitest/config";
+import path from "path";
+
+export default defineConfig({
+  resolve: {
+    alias: {
+      "@": path.resolve(__dirname, "."),
+    },
+  },
+  test: {
+    exclude: ["e2e/**", "node_modules/**"],
+  },
+});