chore(config-ui): modernize frontend toolchain and framework stack

[DoDiODev]

Motivation

Modernizes the config-ui frontend toolchain and framework stack to current
supported versions. This unblocks security updates, removes end-of-life runtimes
and keeps the UI buildable on current Node LTS.

Scope

Coupled framework/runtime upgrade (must land together to keep the build green):

• Runtime/tooling: Node 18 → 24, Yarn 3.4.1 → 4.17.0 (Corepack, vendored
  release swap, .yarnrc.yml), nginx base image bump in config-ui/Dockerfile.
• Build/lint chain: Vite 8, TypeScript config update, migration from
  .eslintrc.js/.eslintignore to ESLint flat config (eslint.config.mjs),
  Vitest setup (vitest.config.ts, vitest.setup.ts) replacing the old
  .file-headerrc.
• UI frameworks: React 18 → 19 and antd 5 → 6, including the resulting
  component/ref/hook fixes and a small miller-columns-select compatibility
  patch (.yarn/patches/...).
• Tests: new src/__tests__/main-entry.test.ts and
  src/plugins/components/__tests__/miller-columns-select.test.tsx.

All changes are limited to config-ui/.

Note on diff size (please read before reviewing)

The raw diff is 42 files, +6,535 / −6,546 lines, but ~96% of that is
generated or vendored and requires no line-by-line review:

Artifact	~Lines	Nature
yarn.lock	~10,621	generated (Yarn 3→4 lockfile migration)
.yarn/releases/yarn-3.4.1.cjs → yarn-4.17.0.cjs	−911 / +944	vendored Yarn binary

The actual hand-written review surface is ~700–900 lines across ~40 small
files: package.json (dependency bumps), eslint.config.mjs (+87, new flat
config), vite.config.ts, vitest.config.ts/setup, Dockerfile (+4), the
antd patch (+34), plus ~25 .tsx/.ts files with 2–37 lines each of
React-19/antd-6 ref/hook/type fixes, and two new test files.

git diff --check flags trailing whitespace only in the two generated/vendored
artifacts above (Yarn binary + patch file); this is expected.

Note on the q-dev connection-field changes

Three files under config-ui/src/plugins/register/q-dev/connection-fields/ are
modified (aws-credentials.tsx, identity-center-config.tsx, s3-config.tsx).
These are required by the React 19 upgrade: React 19 removed the zero-argument
useRef overload, so useRef<string>() must become useRef<string>(undefined),
otherwise tsc --noEmit fails. One file (aws-credentials.tsx) additionally
carries two Prettier-only reformatting hunks (parentheses around ??, JSX
line-wrapping) required for a green eslint .. No behavioral changes.

Automated tests

All run on Node 24.18.0 / Yarn 4.17.0:

• yarn install --immutable → clean (lockfile unchanged)
• yarn exec tsc --noEmit → pass
• yarn exec eslint . → pass (0 errors, 17 pre-existing unused-var warnings)
• yarn build (Vite 8) → pass (1879 modules)
• yarn test (Vitest) → 4/4 passed

Manual test

Rollback

Revert this single commit; no data migrations or backend coupling involved.

Known follow-ups

• Chunk-size warning from Vite (bundle > 500 kB) — pre-existing, out of scope.
• Peer-dependency warnings (React 19 vs. deps still requesting ^18) are benign
  and resolved via the vendored patch / overrides.

[klesh]

Thanks for your contribution. Please fix the linting before we can proceed to review the code.

[DoDiODev]

@klesh should work now