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>

app/styles/globals.css

@@ -2,6 +2,8 @@
 @tailwind components;
 @tailwind utilities;
 
+@source "../../node_modules/streamdown/dist/index.js";
+
 :root {
   --max-width: 1100px;
   --border-radius: 12px;