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)

components/ui/marker.tsx

@@ -0,0 +1,67 @@
+import * as React from "react";
+import { mergeProps } from "@base-ui/react/merge-props";
+import { useRender } from "@base-ui/react/use-render";
+import { cva, type VariantProps } from "class-variance-authority";
+import { cn } from "cn";
+
+const markerVariants = cva(
+  "group/marker relative flex min-h-4 w-full items-center gap-2 text-left text-sm text-muted-foreground [&_svg:not([class*='size-'])]:size-4 [a]:underline [a]:underline-offset-3 [a]:hover:text-foreground",
+  {
+    variants: {
+      variant: {
+        default: "",
+        separator:
+          "before:mr-1 before:h-px before:min-w-0 before:flex-1 before:bg-border after:ml-1 after:h-px after:min-w-0 after:flex-1 after:bg-border",
+        border: "border-b border-border pb-2",
+      },
+    },
+  }
+);
+
+function Marker({
+  className,
+  variant = "default",
+  render,
+  ...props
+}: useRender.ComponentProps<"div"> & VariantProps<typeof markerVariants>) {
+  return useRender({
+    defaultTagName: "div",
+    props: mergeProps<"div">(
+      {
+        className: cn(markerVariants({ variant, className })),
+      },
+      props
+    ),
+    render,
+    state: {
+      slot: "marker",
+      variant,
+    },
+  });
+}
+
+function MarkerIcon({ className, ...props }: React.ComponentProps<"span">) {
+  return (
+    <span
+      data-slot="marker-icon"
+      aria-hidden="true"
+      className={cn("size-4 shrink-0 [&_svg:not([class*='size-'])]:size-4", className)}
+      {...props}
+    />
+  );
+}
+
+function MarkerContent({ className, ...props }: React.ComponentProps<"span">) {
+  return (
+    <span
+      data-slot="marker-content"
+      className={cn(
+        "min-w-0 wrap-break-word group-data-[variant=separator]/marker:flex-none group-data-[variant=separator]/marker:text-center *:[a]:underline *:[a]:underline-offset-3 *:[a]:hover:text-foreground",
+        className
+      )}
+      {...props}
+    />
+  );
+}
+
+export { Marker, MarkerIcon, MarkerContent, markerVariants };