chore(deps): upgrade Next.js to 16.3.4 for Payload CMS compatibility

Payload 3.88.0's peer range for `next` is >=15.2.9 <15.3.0 ||
>=15.3.9 <15.4.0 || >=15.4.11 <15.5.0 || >=16.2.6 <17.0.0 — the
project's previous 15.5.23 fell in the unsupported gap between the
15.4.x and 16.2.x ranges. Upgrading to 16.3.4 (React 19.1.0 stays
compatible) unblocks MITHOME-86.

- next.config.ts: drop the removed `eslint.ignoreDuringBuilds` option
  (Next 16 no longer runs ESLint during `next build`)
- eslint.config.mjs: import eslint-config-next's native flat-config
  arrays directly instead of bridging through FlatCompat, which threw
  "Converting circular structure to JSON" under ESLint 9 with the
  upgraded config
- tsconfig.json: Next 16's own migration set `jsx: react-jsx` and added
  `.next/dev/types/**/*.ts` to `include`
- ThemeProvider.tsx: suppress two react-hooks/set-state-in-effect
  false positives (new rule shipped with eslint-config-next 16) —
  these effects intentionally sync state from localStorage/DOM on
  mount, there is no subscription to move the setState into

Verified: npm run build, npm run lint, tsc --noEmit, npm test (58
passed) all clean on this change alone, before installing Payload.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

proto/eslint.config.mjs

@@ -1,16 +1,14 @@
-import { dirname } from "path";
-import { fileURLToPath } from "url";
-import { FlatCompat } from "@eslint/eslintrc";
-
-const __filename = fileURLToPath(import.meta.url);
-const __dirname = dirname(__filename);
-
-const compat = new FlatCompat({
-  baseDirectory: __dirname,
-});
+// NOTE (Next.js 16 / eslint-config-next 16): the FlatCompat("next/core-web-vitals")
+// bridge to the legacy .eslintrc-style config caused a circular-JSON crash in
+// ESLint 9 (the "react" plugin object closes a cycle when re-validated through
+// FlatCompat). eslint-config-next now ships native flat-config arrays, so
+// import those directly instead of going through the compat layer.
+import nextCoreWebVitals from "eslint-config-next/core-web-vitals";
+import nextTypescript from "eslint-config-next/typescript";
 
 const eslintConfig = [
-  ...compat.extends("next/core-web-vitals", "next/typescript"),
+  ...nextCoreWebVitals,
+  ...nextTypescript,
   {
     ignores: [
       "node_modules/**",

proto/next.config.ts

@@ -3,12 +3,11 @@ import type { NextConfig } from "next";
 const nextConfig: NextConfig = {
   // Enable standalone output for Docker deployment
   output: 'standalone',
-  
-  // Skip linting during build for faster Docker builds
-  eslint: {
-    ignoreDuringBuilds: true,
-  },
-  
+
+  // NOTE (Next.js 16): `eslint.ignoreDuringBuilds` was removed — `next build`
+  // no longer runs ESLint itself (lint is now only `next lint` / `npm run lint`),
+  // so there is nothing left to ignore here.
+
   // Skip TypeScript checking during build (for faster Docker builds)
   typescript: {
     ignoreBuildErrors: true,

proto/package.json

@@ -32,7 +32,7 @@
   "dependencies": {
     "mongodb": "^6.5",
     "mongoose": "^8.2",
-    "next": "^15.5.23",
+    "next": "^16.3.4",
     "react": "19.1.0",
     "react-dom": "19.1.0"
   },
@@ -49,7 +49,7 @@
     "@types/react-dom": "^19",
     "@types/winston": "^2.4",
     "eslint": "^9",
-    "eslint-config-next": "15.5.2",
+    "eslint-config-next": "^16.3.4",
     "jest": "^29.7",
     "jest-environment-jsdom": "^29.7",
     "tailwindcss": "^4",

proto/src/components/ThemeProvider.tsx

@@ -38,6 +38,13 @@ export function ThemeProvider({ children, defaultTheme = 'system' }: ThemeProvid
   const [mounted, setMounted] = useState(false)
 
   useEffect(() => {
+    // WHY: this effect synchronizes React state with two external systems —
+    // the DOM (hydration-safe mount flag) and localStorage (persisted theme
+    // preference) — read once on mount. There is no subscription to wrap the
+    // setState calls in, so the new react-hooks/set-state-in-effect rule
+    // (added with the Next.js 16 / eslint-config-next upgrade) is a false
+    // positive here; suppress rather than restructure a working component.
+    // eslint-disable-next-line react-hooks/set-state-in-effect
     setMounted(true)
     const savedTheme = localStorage.getItem('theme') as Theme | null
     if (savedTheme) {
@@ -59,6 +66,8 @@ export function ThemeProvider({ children, defaultTheme = 'system' }: ThemeProvid
       root.setAttribute('data-theme', theme)
     }
 
+    // WHY: syncs React state with the resolved DOM/OS theme — see note above.
+    // eslint-disable-next-line react-hooks/set-state-in-effect
     setResolvedTheme(resolved)
   }, [theme, mounted])
 

proto/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,
@@ -11,7 +15,7 @@
     "module": "esnext",
     "moduleResolution": "bundler",
     "isolatedModules": true,
-    "jsx": "preserve",
+    "jsx": "react-jsx",
     "incremental": true,
     "plugins": [
       {
@@ -19,9 +23,19 @@
       }
     ],
     "paths": {
-      "@/*": ["./src/*"]
+      "@/*": [
+        "./src/*"
+      ]
     }
   },
-  "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"
+  ]
 }