Upgrade to Next.js 16.1.6, React 19.2.4, and update all dependencies

- Next.js 15.5 → 16.1.6 (Turbopack default, faster builds)
- React/React-DOM 19.1 → 19.2.4
- @types/react 18 → 19, @types/react-dom 18 → 19
- TypeScript 5.2 → 5.9
- CI Node.js 18 → 20 (required by Next.js 16)
- Replace removed next lint with standalone ESLint config
- Auto-updated tsconfig.json (jsx: react-jsx, dev types)

.eslintrc.json

@@ -0,0 +1,15 @@
+{
+  "extends": ["eslint:recommended", "plugin:@typescript-eslint/recommended"],
+  "parser": "@typescript-eslint/parser",
+  "parserOptions": {
+    "ecmaVersion": "latest",
+    "sourceType": "module",
+    "ecmaFeatures": { "jsx": true }
+  },
+  "plugins": ["@typescript-eslint"],
+  "rules": {
+    "@typescript-eslint/no-unused-vars": ["warn", { "argsIgnorePattern": "^_" }],
+    "@typescript-eslint/no-explicit-any": "warn"
+  },
+  "ignorePatterns": ["node_modules/", "out/", ".next/", "api/"]
+}