Upgrade jest-dom and webpack-dev-server to next major

[rzueger]

First batch of the remaining major upgrades, scoped to packages that are dev/test-only and do not ship in the bundle.

Changes

• @testing-library/jest-dom 6 → 7. Used only by test/testSetup.js and global.d.ts. No deprecated matchers were in use, so no source changes were needed.
• webpack-dev-server 5 → 6. webpack.config.js has no devServer block, so there was no configuration surface to migrate.

Only package.json and package-lock.json change.

Verification

Against a clean tree, in the same order the CI build job runs:

• npm run typecheck — clean
• npm run build — succeeds
• npm run test:app — 2057/2057 pass, 176/176 suites, 9/9 snapshots

For webpack-dev-server, which none of the above exercises, I booted it directly (npm start --project=cypress-testing): it compiles, listens on 8080, serves index.html with the expected cypress-testing title, and serves the emitted bundle (HTTP 200, 12.7 MB). That is the same path CI's cy:test drives via start-server-and-test.

npm run cy:test itself was not run locally — the Cypress binary download is blocked in my environment — but CI covers it.

Deliberately not included

@firebase/rules-unit-testing 3 → 5, though it is also dev-only. It is used solely by test/rules/movementWrite.rules.js, which runs through npm run test:rules and is not part of CI (build and build_functions are the only checks). Verifying it needs the Firebase emulator, and the Firebase CLI is neither installed nor a project dependency here. Upgrading it in this PR would mean landing it with neither CI nor local coverage, so it should go in its own change where someone can run npm run test:rules once locally.