fix: downgrade ESLint to v8.57.1 for LSP compatibility

- Downgrade ESLint from v10.1.0 to v8.57.1
- Create .eslintrc.json config (ESLint v8 format)
- Remove eslint.config.js (flat config format)
- ESLint LSP (vscode-langservers-extracted) doesn't support ESLint v10+

The vscode-eslint-language-server from vscode-langservers-extracted v4.10.0
is not compatible with ESLint v10's Flat Config format. Downgrading to
ESLint v8 ensures the ESLint LSP in Neovim works correctly.

.eslintrc.json

@@ -0,0 +1,20 @@
+{
+  "env": {
+    "browser": true,
+    "es2021": true,
+    "node": true
+  },
+  "parser": "@typescript-eslint/parser",
+  "parserOptions": {
+    "ecmaVersion": "latest",
+    "sourceType": "module"
+  },
+  "plugins": ["@typescript-eslint"],
+  "ignorePatterns": [
+    ".next",
+    "out",
+    "build",
+    "next-env.d.ts",
+    "src/app/test-api/**"
+  ]
+}