feat: update Next.js configuration and package dependencies

- Adjusted `reactStrictMode` in `next.config.ts` to reduce strict mode double mounts during development, addressing hydration issues.
- Modified `dev` script in `package.json` to suppress hydration warnings, and added a `dev:strict` script for strict mode development.
- Added new dependencies: `@polar-sh/better-auth` and `@polar-sh/sdk` for enhanced authentication and SDK functionalities.
- Updated `pnpm-lock.yaml` to reflect new package versions and dependencies.

next.config.ts

@@ -1,6 +1,11 @@
 import type { NextConfig } from "next";
 
 const nextConfig: NextConfig = {
+  // Reduziert in der Entwicklung Strict-Mode-Doppel-Mounts (häufige Ursache für
+  // „Hydration“-Lärm). Echte Server/Client-Mismatches können weiterhin auftreten;
+  // dann `pnpm dev:strict` zum Debuggen oder Ursache beheben.
+  reactStrictMode:
+    process.env.NEXT_DEV_SUPPRESS_HYDRATION === "1" ? false : null,
   images: {
     remotePatterns: [
       {