refactor: load env files with @next/env instead of dotenv

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

content-collections.ts

@@ -1,5 +1,5 @@
-import "dotenv/config";
 import { defineCollection, defineConfig } from "@content-collections/core";
+import nextEnv from "@next/env";
 import { decode } from "html-entities";
 import { unified } from "unified";
 import { z } from "zod";
@@ -14,6 +14,8 @@ import {
   remarkStripMdxImportsExports,
 } from "@/lib/remark";
 
+nextEnv.loadEnvConfig(process.cwd());
+
 const POSTS_DIR = "notes" as const;
 
 const parseableDate = z.string().refine((value) => !Number.isNaN(Date.parse(value)), {