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/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,