EsLint v9 (#154)

* Upgrade ESLint and related configurations to version 9

- Update ESLint to version 9.19.0
- Migrate ESLint configurations to flat config format
- Remove deprecated ESLint config files
- Update package dependencies and configurations
- Simplify ESLint setup across packages
- Remove unnecessary ESLint config blocks from package.json files
- Improved CI caching with Turborepo tasks
- Removed duplicate styles

apps/web/postcss.config.mjs

@@ -0,0 +1,5 @@
+export default {
+  plugins: {
+    '@tailwindcss/postcss': {},
+  },
+};