Upgrade webpack-dev-server 5.2.6 → 6.0.0

# Upgrade webpack-dev-server 5.2.6 → 6.0.0 (major)

Dependabot PR #7754. Dev dependency (`devDependencies`, currently `^5.2.1`, lockfile 5.2.5); powers the local `npm run serve` dev server only, not shipped to production. Pulled out of the weekly Dependabot slot per the major-version process — needs an owner and verification, not a batch merge.

## Compatibility pre-check

- ✅ Node: v6 requires `>= 22.15.0`; dev-only tool and the dev/CI default is Node 24, which satisfies it. Shipped `engines: >=20.10.0` is unaffected (not a runtime dep). Ensure CI build lanes run Node ≥ 22.15.
- ✅ webpack: v6 peer `^5.101.0`; installed `webpack` 5.108.4 satisfies it.
- ✅ webpack-cli: installed 7.2.1; v6 removes built-in CLI flags in favor of `webpack-cli serve` (already used).

## Breaking changes to review

- Drops Node.js < 22.15.0.
- Bumps webpack peer from `^5.0.0` to `^5.101.0`.
- Removes CLI flags — use the `serve` command from `webpack-cli`.
- Removes the `bypass` proxy option.
- Removes `internalIP`/`internalIPSync` static methods on Server.
- Drops SockJS: `webSocketServer` no longer accepts `'sockjs'`.
- Removes `spdy`; HTTP/2 now via Node `node:http2`.
- Express 5, `http-proxy-middleware` v4, `webpack-dev-middleware` v8 (`getFilenameFromUrl()` now async).
- Our config uses only `devServer.port` and `historyApiFallback` — neither is listed as changed; to confirm from changelog.

## To do before merge

- [ ] Verify `npm run serve` (dev server) still boots and HMR works.
- [ ] Reconcile any renamed/removed devServer options with our config.
- [ ] Read the 6.x migration guide.

## Refs

- Branch: `dependabot/npm_and_yarn/webpack-dev-server-6.0.0`
- PR: #7754
- Upstream: https://github.com/webpack/webpack-dev-server/releases/tag/v6.0.0

Start with Dependabot PR #7754 and the branch named in the issue, then read the webpack-dev-server 6.x migration guide and release changelog. Run npm run serve to verify that the dev server boots and HMR works; done means the configuration remains compatible and CI build lanes use Node 22 or newer.

Written by the indexing model from the issue text.