feat: setup full Next.js toolchain and core dependencies

- Add runtime deps: react-query, react-hook-form+zod, zustand, ky, openapi-fetch,
  better-auth, react-markdown, shiki, recharts, next-themes, nuqs, sonner, cmdk,
  vaul, embla-carousel-react, motion, date-fns, tailwind-merge, clsx, cva
- Add dev tooling: vitest, testing-library, playwright, prettier(+tailwind plugin),
  husky, lint-staged, jsdom
- Wire providers (QueryClient, Theme, NuqsAdapter, Tooltip, Sonner) into layout
- Add vitest/playwright/prettier configs + lint-staged pre-commit hook
- Add lib/query-client + lib/http (ky) helpers and sample unit test
- Fix react-hooks/set-state-in-effect lint errors (use-mobile useSyncExternalStore,
  carousel async init)
- Remove committed Next-generated types/ duplicates (regenerated under .next/types)

lib/__tests__/cn.test.ts

@@ -0,0 +1,19 @@
+import { describe, expect, it } from "vitest";
+
+export function cn(...classes: Array<string | false | null | undefined>): string {
+  return classes.filter(Boolean).join(" ");
+}
+
+describe("cn", () => {
+  it("joins truthy values", () => {
+    expect(cn("a", "b", "c")).toBe("a b c");
+  });
+
+  it("drops falsy values", () => {
+    expect(cn("a", false, null, undefined, "b")).toBe("a b");
+  });
+
+  it("returns empty string for no classes", () => {
+    expect(cn()).toBe("");
+  });
+});

lib/http.ts

@@ -0,0 +1,27 @@
+import ky from "ky";
+
+/**
+ * Typed HTTP client built on ky.
+ * Use openapi-fetch alongside this when you have a generated OpenAPI
+ * schema (see openapi-fetch docs: `createClient<paths>({ baseUrl })`).
+ */
+export const apiClient = ky.create({
+  prefix: "/api",
+  timeout: 15_000,
+  retry: {
+    limit: 2,
+    methods: ["get"],
+    statusCodes: [408, 429, 500, 502, 503, 504],
+  },
+});
+
+export const http = {
+  get: <T>(url: string, init?: RequestInit) => apiClient.get(url, init).json<T>(),
+  post: <T>(url: string, body?: unknown, init?: RequestInit) =>
+    apiClient.post(url, { json: body, ...init }).json<T>(),
+  put: <T>(url: string, body?: unknown, init?: RequestInit) =>
+    apiClient.put(url, { json: body, ...init }).json<T>(),
+  patch: <T>(url: string, body?: unknown, init?: RequestInit) =>
+    apiClient.patch(url, { json: body, ...init }).json<T>(),
+  delete: <T>(url: string, init?: RequestInit) => apiClient.delete(url, init).json<T>(),
+};

lib/query-client.ts

@@ -0,0 +1,28 @@
+import { isServer, QueryClient } from "@tanstack/react-query";
+
+function makeQueryClient() {
+  return new QueryClient({
+    defaultOptions: {
+      queries: {
+        staleTime: 60 * 1000,
+        gcTime: 5 * 60 * 1000,
+        refetchOnWindowFocus: false,
+        retry: 1,
+      },
+    },
+  });
+}
+
+let browserQueryClient: QueryClient | undefined = undefined;
+
+export function getQueryClient() {
+  if (isServer) {
+    // Server: fresh client per request so caches never leak between users.
+    return makeQueryClient();
+  }
+  // Browser: single shared client so the cache survives route changes.
+  if (!browserQueryClient) {
+    browserQueryClient = makeQueryClient();
+  }
+  return browserQueryClient;
+}

lib/utils.ts

@@ -0,0 +1 @@
+export { cn } from "cn";