fix(deps): align vitest and stylelint so npm ci resolves again (#422)

Two dependabot bumps landed on development on 2026-08-30 without CI, and each
leaves the tree unresolvable:

- @vitest/coverage-v8 went to 4.1.11 while vitest and @vitest/ui stayed on
  ^3.2.7, so coverage-v8 peers vitest 4.1.11 against @vitest/ui's vitest 3.2.7
- stylelint went to 17.14.1, which @nextcloud/stylelint-config 2.4.0 cannot
  peer: it wants ^15.6.0

Both are reverted to the version the rest of their own ecosystem is on, rather
than bumping the ecosystem, because a vitest 3 to 4 move is a separate change
that deserves its own testing.

Verified with npm ci --dry-run from the committed lock: exit 0.

Co-authored-by: Conduction Release Bot <release-bot@conduction.nl>

Author: rubenvdlinde