feat: upgrade to Next.js 15, React 19, and migrate to ESLint 9 flat config

Major framework upgrades:
- Next.js 13 → 15.5.6
- React 18.2.0 → 19.2.0
- ESLint 8 → 9 with flat config migration

Dependency updates:
- @headlessui/react, date-fns, zod, true-myth, usehooks-ts
- @typescript-eslint packages to v8
- TypeScript to v5.9.3
- Prettier to v3.6.2

Breaking changes:
- Migrate from .eslintrc.json to eslint.config.mjs
- Remove deprecated swcMinify option from next.config.mjs
- Update all React type definitions

package.json

@@ -5,7 +5,7 @@
   "scripts": {
     "build": "next build",
     "dev": "next dev",
-    "lint": "next lint",
+    "lint": "eslint .",
     "start": "next start",
     "test": "vitest",
     "test:ui": "vitest --ui",
@@ -15,43 +15,41 @@
     "type-check": "tsc --noEmit"
   },
   "dependencies": {
-    "@headlessui/react": "^2.0.3",
+    "@headlessui/react": "^2.2.9",
     "@heroicons/react": "^2.0.16",
     "@swc/helpers": "^0.5.11",
     "clsx": "^2.1.1",
-    "date-fns": "^2.29.3",
+    "date-fns": "^4.1.0",
     "immutability-helper": "^3.1.1",
-    "next": "^13",
-    "react": "18.2.0",
-    "react-dom": "18.2.0",
+    "next": "^15.5.6",
+    "react": "19.2.0",
+    "react-dom": "19.2.0",
     "react-hook-form": "^7.42.1",
-    "react-timeago": "^7.2.0",
+    "react-timeago": "^8.3.0",
     "sass": "^1.57.1",
-    "true-myth": "^7.1.0",
-    "usehooks-ts": "^2.9.1",
-    "zod": "^3.20.2"
+    "true-myth": "^9.2.0",
+    "usehooks-ts": "^3.1.1",
+    "zod": "^4.1.12"
   },
   "devDependencies": {
     "@testing-library/jest-dom": "^6.9.1",
     "@testing-library/react": "^16.3.0",
-    "@types/node": "^18.11.18",
-    "@types/prettier": "^2.7.2",
-    "@types/react": "^18.0.26",
-    "@types/react-dom": "^18.0.10",
-    "@types/react-timeago": "^4.1.7",
-    "@typescript-eslint/eslint-plugin": "^5.47.1",
-    "@typescript-eslint/parser": "^5.47.1",
+    "@types/node": "^24.9.1",
+    "@types/react": "^19.2.2",
+    "@types/react-dom": "^19.2.2",
+    "@typescript-eslint/eslint-plugin": "^8.46.2",
+    "@typescript-eslint/parser": "^8.46.2",
     "@vitest/ui": "^3.2.4",
     "autoprefixer": "^10.4.7",
-    "eslint": "^8.30.0",
-    "eslint-config-next": "13.1.1",
+    "eslint": "^9.38.0",
+    "eslint-config-next": "15.5.6",
     "happy-dom": "^20.0.8",
     "postcss": "^8.4.14",
-    "prettier": "^2.8.8",
-    "prettier-plugin-tailwindcss": "^0.2.1",
+    "prettier": "^3.6.2",
+    "prettier-plugin-tailwindcss": "^0.7.1",
     "tailwindcss": "^3.2.0",
-    "type-fest": "^4.18.2",
-    "typescript": "^4.9.4",
+    "type-fest": "^5.1.0",
+    "typescript": "^5.9.3",
     "vitest": "^3.2.4"
   },
   "ct3aMetadata": {