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/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",