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/playwright.config.ts

@@ -0,0 +1,20 @@
+import { defineConfig, devices } from "@playwright/test";
+
+export default defineConfig({
+  testDir: "./e2e",
+  fullyParallel: true,
+  retries: 0,
+  reporter: "html",
+  use: {
+    baseURL: "http://localhost:3000",
+    trace: "on-first-retry",
+  },
+  projects: [
+    { name: "chromium", use: { ...devices["Desktop Chrome"] } },
+  ],
+  webServer: {
+    command: "npm run dev",
+    url: "http://localhost:3000",
+    reuseExistingServer: true,
+  },
+});