Adds automated checks that were previously only run manually and locally, closing the last remaining gap before dev can be merged into main with confidence. Runs on every push and pull request targeting dev or main: - lint (eslint) - format check (prettier --check) - changeset presence check (non-blocking for now, see comment in workflow file) - unit tests (vitest) - component/browser tests (vitest + playwright, via the existing Docker test setup) Fixes: #10