The lint script referenced eslint with --ext flags but the repo never had
an ESLint config file, so pnpm lint always failed. Add a flat config
(typescript-eslint recommended + vue essential) with legacy-code rules
downgraded to warnings, drop the flat-config-incompatible --ext flags,
and move pnpm build approvals from the no-longer-read
pnpm.onlyBuiltDependencies field to pnpm-workspace.yaml allowBuilds.