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/**",