test: stabilize frontend quality gates

[ichwars]

Summary

• cap Vitest workers to prevent local OOM and spawn failures
• disable Node's experimental worker Web Storage in favor of jsdom
• clean up mounted providers before restoring mocks and locale
• enforce the current frontend coverage baseline in CI

Coverage baseline

• statements: 46.04% (gate 46%)
• branches: 41.85% (gate 41%)
• functions: 37.55% (gate 37%)
• lines: 46.95% (gate 46%)

Validation

• 181 test files / 2,466 tests passed
• coverage thresholds passed
• i18n parity passed
• ESLint passed
• TypeScript production build passed