test: add component unit tests for critical UI components (Closes #535)

[waterWang]

Summary

Add component unit tests for 5 critical UI components as specified in #535.

Changes

• vitest.config.ts — standalone vitest config with jsdom environment, test setup, and CSS modules support
• src/test/setup.ts — test setup importing jest-dom matchers, clearing localStorage between tests, and stubbing matchMedia/scrollTo for jsdom
• src/components/tests/ErrorBoundary.test.tsx — 3 tests: renders children, shows fallback on error, captures exception via Sentry
• src/components/tests/CSVUploader.test.tsx — 8 tests: renders upload zone, parses valid CSV, validates required columns, applies custom validators, rejects non-CSV files, supports drag-and-drop, shows row counts, opens file picker
• src/components/tests/ConnectAccount.test.tsx — 7 tests: disconnected state, connected state with address, Social Active badge, connecting spinner, Sign In navigation, Connect Wallet click, Exit disconnect
• src/components/tests/EmployeeList.test.tsx — 10 tests: renders table, empty state, wallet shortening, mobile card view, salary edit modal, delete confirmation, CSV import flow, cancellation
• src/components/tests/BulkPaymentStatusTracker.test.tsx — 11 tests: mount with fetch, empty state, data rendering, error state, refresh, expand recipient details, Retry Failed button, socket setup, batch subscription, cleanup on unmount
• eslint.config.js — relax unsafe-type rules scoped to test files
• package.json — add test and test:watch scripts

Dev Dependencies Added

• @testing-library/react ^16.3.2
• @testing-library/jest-dom ^7.0.1
• @testing-library/user-event ^14.6.6
• jsdom ^30.0.1

Verification

cd frontend
pnpm test
# → 39 tests passed across 5 test files

Closes #535