fix(frontend): upgrade Docker Node.js from v21 (EOL) to v22 LTS (#12561)

## Summary
Upgrade the frontend **Docker image** from **Node.js v21** (EOL since
June 2024) to **Node.js v22 LTS** (supported through April 2027).

> **Scope:** This only affects the **Dockerfile** used for local
development (`docker compose`) and CI. It does **not** affect Vercel
(which manages its own Node.js runtime) or Kubernetes (the frontend Helm
chart was removed in Dec 2025 — the frontend is deployed exclusively via
Vercel).

## Why
- Node v21.7.3 has a **known TransformStream race condition bug**
causing `TypeError: controller[kState].transformAlgorithm is not a
function` — this is
[BUILDER-3KF](https://significant-gravitas.sentry.io/issues/BUILDER-3KF)
with **567,000+ Sentry events**
- The error is entirely in Node.js internals
(`node:internal/webstreams/transformstream`), zero first-party code
- Node 21 is **not an LTS release** and has been EOL since June 2024
- `package.json` already declares `"engines": { "node": "22.x" }` — the
Dockerfile was inconsistent
- Node 22.x LTS (v22.22.1) fixes the TransformStream bug
- Next.js 15.4.x requires Node 18.18+, so Node 22 is fully compatible

## Changes
- `autogpt_platform/frontend/Dockerfile`: `node:21-alpine` →
`node:22.22-alpine3.23` (both `base` and `prod` stages)

## Test plan
- [ ] Verify frontend Docker image builds successfully via `docker
compose`
- [ ] Verify frontend starts and serves pages correctly in local Docker
environment
- [ ] Monitor Sentry for BUILDER-3KF — should drop to zero for
Docker-based runs