Add React Testing Library framework for frontend component testing

[Copilot]

Summary

Establishes a component testing framework for the frontend using React Testing Library + Vitest/jsdom. Previously, tests covered only pure logic (utilities, reducers, graph construction) — no DOM or component-level tests were possible.

Infrastructure

• Add @testing-library/react, @testing-library/user-event, @testing-library/jest-dom, jsdom as dev deps
• Configure Vitest with environment: 'jsdom' and a global setup file in vite.config.mts

src/tests/setup.ts

• Extends Vitest's expect with jest-dom DOM matchers (toBeInTheDocument, toHaveAttribute, etc.)
• Registers afterEach(cleanup) to prevent DOM leaking between tests

src/tests/test-utils.tsx — reusable render wrapper

// Wraps component with Redux store + ChakraProvider; returns render result + store
const { getByText, store } = renderWithProviders(<MyComponent />, { store: prebuiltStore });

Accepts an optional store parameter for tests that need pre-seeded Redux state.

Example tests

• WavyLine.test.tsx — pure component, uses render directly; demonstrates SVG attribute assertions
• IAIImageFallback.test.tsx — Chakra UI component, uses renderWithProviders; demonstrates label/icon conditional rendering tests

Related Issues / Discussions

Closes #5440

QA Instructions

cd invokeai/frontend/web
pnpm test:no-watch
# 34 test files, 755 tests — all pass

New test files can be verified individually:

pnpm test:no-watch src/common/components/WavyLine.test.tsx
pnpm test:no-watch src/common/components/IAIImageFallback.test.tsx

Merge Plan

No special merge considerations.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

[lstein]

Still thinking about this...