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)

tsconfig.json

@@ -1,7 +1,11 @@
 {
   "compilerOptions": {
     "target": "ES2017",
-    "lib": ["dom", "dom.iterable", "esnext"],
+    "lib": [
+      "dom",
+      "dom.iterable",
+      "esnext"
+    ],
     "allowJs": true,
     "skipLibCheck": true,
     "strict": true,
@@ -12,7 +16,7 @@
     "moduleResolution": "bundler",
     "resolveJsonModule": true,
     "isolatedModules": true,
-    "jsx": "preserve",
+    "jsx": "react-jsx",
     "incremental": true,
     "plugins": [
       {
@@ -20,9 +24,19 @@
       }
     ],
     "paths": {
-      "@/*": ["./*"]
+      "@/*": [
+        "./*"
+      ]
     }
   },
-  "include": ["next-env.d.ts", "**/*.ts", "**/*.tsx", ".next/types/**/*.ts"],
-  "exclude": ["node_modules"]
-}
+  "include": [
+    "next-env.d.ts",
+    "**/*.ts",
+    "**/*.tsx",
+    ".next/types/**/*.ts",
+    ".next/dev/types/**/*.ts"
+  ],
+  "exclude": [
+    "node_modules"
+  ]
+}