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/client.ts

@@ -0,0 +1,110 @@
+import type {
+  Dashboard,
+  Group,
+  Service,
+  WidgetInstance,
+  WidgetData,
+  AssetFile,
+  CreateGroupRequest,
+  PatchGroupRequest,
+  ServiceRequest,
+  LayoutRequest,
+  WidgetRequest,
+} from "./schema";
+
+const API_BASE = process.env.NEXT_PUBLIC_API_BASE_URL || "http://localhost:8080";
+
+async function request<T>(path: string, init?: RequestInit): Promise<T> {
+  const res = await fetch(`${API_BASE}${path}`, {
+    headers: { "Content-Type": "application/json", ...init?.headers },
+    ...init,
+  });
+  if (res.status === 204) return undefined as T;
+  if (!res.ok) {
+    const err = await res.json().catch(() => ({ message: res.statusText }));
+    throw new ApiError(res.status, err.code || "unknown", err.message || "Request failed", err.details);
+  }
+  return res.json();
+}
+
+export class ApiError extends Error {
+  constructor(
+    public status: number,
+    public code: string,
+    message: string,
+    public details?: unknown,
+  ) {
+    super(message);
+    this.name = "ApiError";
+  }
+}
+
+// ── Dashboard ──
+export function getDashboard(): Promise<Dashboard> {
+  return request("/api/v1/dashboard");
+}
+
+// ── Groups ──
+export function listGroups(): Promise<Group[]> {
+  return request("/api/v1/groups");
+}
+export function createGroup(body: CreateGroupRequest): Promise<Group> {
+  return request("/api/v1/groups", { method: "POST", body: JSON.stringify(body) });
+}
+export function patchGroup(id: string, body: PatchGroupRequest): Promise<Group> {
+  return request(`/api/v1/groups/${id}`, { method: "PATCH", body: JSON.stringify(body) });
+}
+export function deleteGroup(id: string, moveServices = false): Promise<void> {
+  return request(`/api/v1/groups/${id}?moveServicesToUngrouped=${moveServices}`, { method: "DELETE" });
+}
+
+// ── Services ──
+export function listServices(): Promise<Service[]> {
+  return request("/api/v1/services");
+}
+export function createService(body: ServiceRequest): Promise<Service> {
+  return request("/api/v1/services", { method: "POST", body: JSON.stringify(body) });
+}
+export function patchService(id: string, body: ServiceRequest): Promise<Service> {
+  return request(`/api/v1/services/${id}`, { method: "PATCH", body: JSON.stringify(body) });
+}
+export function deleteService(id: string): Promise<void> {
+  return request(`/api/v1/services/${id}`, { method: "DELETE" });
+}
+
+// ── Layout ──
+export function putLayout(body: LayoutRequest): Promise<Dashboard> {
+  return request("/api/v1/layout", { method: "PUT", body: JSON.stringify(body) });
+}
+
+// ── Assets ──
+export async function uploadIcon(file: File): Promise<AssetFile> {
+  const form = new FormData();
+  form.append("file", file);
+  const res = await fetch(`${API_BASE}/api/v1/assets/icons`, { method: "POST", body: form });
+  if (!res.ok) {
+    const err = await res.json().catch(() => ({ message: res.statusText }));
+    throw new ApiError(res.status, err.code || "unknown", err.message || "Upload failed");
+  }
+  return res.json();
+}
+
+// ── Widgets ──
+export function listWidgets(): Promise<WidgetInstance[]> {
+  return request("/api/v1/widgets");
+}
+export function createWidget(body: WidgetRequest): Promise<WidgetInstance> {
+  return request("/api/v1/widgets", { method: "POST", body: JSON.stringify(body) });
+}
+export function patchWidget(id: string, body: WidgetRequest): Promise<WidgetInstance> {
+  return request(`/api/v1/widgets/${id}`, { method: "PATCH", body: JSON.stringify(body) });
+}
+export function deleteWidget(id: string): Promise<void> {
+  return request(`/api/v1/widgets/${id}`, { method: "DELETE" });
+}
+export function getWidgetData(id: string): Promise<WidgetData> {
+  return request(`/api/v1/widgets/${id}/data`);
+}
+export function refreshWidget(id: string): Promise<WidgetData> {
+  return request(`/api/v1/widgets/${id}/refresh`, { method: "POST" });
+}