fix: surface batch operation failures

[itzptk]

Summary

• centralizes batch-operation toast copy for upgrade/uninstall/pin actions
• emits an error toast when any batch items fail while keeping success toasts for all-success batches
• adds Angular service coverage for partial/all-failure batch results

Closes #7

Verification

• npm test -- --include src/app/core/services/package-actions.service.spec.ts
• npm run test:all
• npm run build
• independent code review: passed; no blocking findings