fix: resolve "circular structure" error in ESLint & complete Flat Config setup

[utact]

Summary

This PR fixes the TypeError: Converting circular structure to JSON error affecting local linting and CI, and completes the migration to ESLint 9's Flat Config format with full TypeScript support.

Problem Analysis

The current eslint.config.mjs uses FlatCompat to extend next/core-web-vitals.
In the current version of eslint-config-next + ESLint 9, FlatCompat creates a circular reference within the internal plugin configuration (specifically related to react-hooks), causing the config validator to crash with a JSON serialization error.

Additionally, the previous configuration relied on the default parser, which could cause parsing errors in TypeScript files (.ts, .tsx).

$ npx eslint .

Oops! Something went wrong! :(

ESLint: 9.39.1

TypeError: Converting circular structure to JSON
    --> starting at object with constructor 'Object'
    |     property 'configs' -> object with constructor 'Object'
    |     property 'flat' -> object with constructor 'Object'
    |     ...
    |     property 'plugins' -> object with constructor 'Object'
    --- property 'react' closes the circle

ConfigData (node_modules/@eslint/eslintrc/lib/config-array-factory.js:676:19)
    at ConfigArrayFactory._loadConfigData (node_modules/@eslint/eslintrc/lib/config-array-factory.js:641:21)

Solution

I have refactored eslint.config.mjs to use a native Flat Config approach (Plugin Composition) instead of FlatCompat.

1. Removed FlatCompat: Avoids the circular dependency issue entirely by manually composing plugins.
2. Added typescript-eslint: Integrated tseslint.config() to ensure proper parsing and type-checking for TypeScript files.
3. Updated Script: Changed "lint": "next lint" to "lint": "eslint ." in package.json (aligned with Next.js 16 breaking changes).
4. Explicit Dependencies: Added @next/eslint-plugin-next, eslint-plugin-react, etc., to devDependencies to support the new config structure.

Verification

• Ran npm run lint locally -> Passed (No circular errors, correct linting).
• Verified that .next and node_modules are correctly ignored.

This change should resolve the linting issues in the CI pipeline as well.

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