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/tests/utils.test.ts

@@ -0,0 +1,16 @@
+import { describe, it, expect } from "vitest";
+import { cn } from "@/lib/utils";
+
+describe("cn utility", () => {
+  it("merges class names", () => {
+    expect(cn("foo", "bar")).toBe("foo bar");
+  });
+
+  it("handles conditional classes", () => {
+    expect(cn("foo", false && "bar", "baz")).toBe("foo baz");
+  });
+
+  it("deduplicates tailwind classes", () => {
+    expect(cn("px-2", "px-4")).toBe("px-4");
+  });
+});