feat: migrate ESLint to v9 flat config

- Replace .eslintrc.json with eslint.config.mjs
- Simplify configuration using @antfu/eslint-config
- Add necessary ESLint plugin dependencies
- Disable overly strict style rules
- Set package.json type to module for ESM support
- Fix ESLint disable comment format

🤖 Generated with [Claude Code](https://claude.ai/code)

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

utils/string.ts

@@ -1,6 +1,6 @@
 const chars = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ-_'
 export function randomString(length: number) {
   let result = ''
-  for (let i = length; i > 0; --i) result += chars[Math.floor(Math.random() * chars.length)]
+  for (let i = length; i > 0; --i) { result += chars[Math.floor(Math.random() * chars.length)] }
   return result
 }