test: add Vitest and Playwright testing infrastructure (#512)

* test: add Vitest and Playwright testing infrastructure

- Add Vitest for unit tests (39 tests)
  - cached-responses.test.ts
  - ai-providers.test.ts
  - chat-helpers.test.ts
  - utils.test.ts
- Add Playwright for E2E tests (3 smoke tests)
  - Homepage load
  - Japanese locale
  - Settings dialog
- Add CI workflow (.github/workflows/test.yml)
- Add vitest.config.mts and playwright.config.ts
- Update .gitignore for test artifacts

* test: add more E2E tests for UI components

- Chat panel tests (interactive elements, iframe)
- Settings tests (dark mode, language, draw.io theme)
- Save dialog tests (buttons exist)
- History dialog tests
- Model config tests
- Keyboard interaction tests
- Upload area tests

Total: 15 E2E tests, all passing

* test: fix E2E test issues from review

Fixes based on Gemini and Codex review:
- Remove brittle nth(1) selector in keyboard tests
- Remove waitForTimeout(500) race condition
- Remove if(isVisible) silent skip patterns
- Add proper assertions instead of no-op checks
- Remove expect(count >= 0) that always passes
- Remove unused hasProviderUI variable

All 14 E2E tests and 39 unit tests pass.

* style: auto-format with Biome

* fix: resolve lint errors for CI

* test(e2e): add diagram generation tests with mocked AI responses

- Add tests for generate, edit, and append diagram operations
- Use SSE mocked responses matching AI SDK UI message stream format
- Generate mxCell XML directly in tests for deterministic assertions
- Tests verify tool card rendering and 'Complete' badge state

* test: add comprehensive E2E tests for all major features

- Error handling tests (API errors, rate limits, network timeout, truncated XML)
- Multi-turn conversation tests (sequential requests, history preservation)
- File upload tests (upload button, file preview, sending with message)
- Theme switching tests (dark mode toggle, persistence, system preference)
- Language switching tests (EN/JA/ZH, persistence, locale URLs)
- Iframe interaction tests (draw.io loading, toolbar, diagram rendering)
- Copy/paste tests (chat input, XML input, special characters)
- History restore tests (new chat, persistence, browser navigation)

* refactor: extract shared test helpers and improve error assertions

- Create tests/e2e/lib/helpers.ts with shared SSE mock functions
- Add proper error UI assertions to error-handling.spec.ts
- Remove waitForTimeout calls in favor of real assertions
- Update 6 test files to use shared helpers

* docs: add testing section to CONTRIBUTING.md

* fix: improve test infrastructure based on PR review

- Fix double build in CI: remove redundant build from playwright webServer
- Export chat helpers from shared module for proper unit testing
- Replace waitForTimeout with explicit waits in E2E tests
- Add data-testid attributes to settings and new chat buttons
- Add list reporter for CI to show failures in logs
- Add Playwright browser caching to speed up CI
- Add vitest coverage configuration
- Fix conditional test assertions to use test.skip() instead of silent pass
- Remove unused variables flagged by linter

* fix: improve E2E test assertions and remove silent skips

- Replace silent test.skip() with explicit conditional skips
- Add actual persistence assertion after page reload
- Use data-testid selector for new chat button test

* refactor: add shared fixtures and test.step() patterns

- Add tests/e2e/lib/fixtures.ts with shared test helpers
- Add tests/e2e/fixtures/diagrams.ts with XML test data
- Add expectBeforeAndAfterReload() helper for persistence tests
- Add test.step() for better test reporting in complex tests
- Consolidate mock helpers into fixtures module
- Reduce code duplication across 17 test files

* fix: make persistence tests more reliable

- Remove expectBeforeAndAfterReload from mocked API tests
- Add explicit test.step() for before/after reload checks
- Add retry config for flaky clipboard tests
- Add sleep after reload for language persistence test

* test: remove flaky XML paste test

* docs: run both unit and e2e tests before PR

* chore: add type check and unit test git hooks

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

.github/workflows/test.yml

@@ -0,0 +1,75 @@
+name: Test
+
+on:
+  pull_request:
+    branches: [main]
+  push:
+    branches: [main]
+
+jobs:
+  lint-and-unit:
+    name: Lint & Unit Tests
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: "20"
+          cache: "npm"
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Run lint
+        run: npm run check
+
+      - name: Run unit tests
+        run: npm run test -- --run
+
+  e2e:
+    name: E2E Tests
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: "20"
+          cache: "npm"
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Cache Playwright browsers
+        uses: actions/cache@v4
+        id: playwright-cache
+        with:
+          path: ~/.cache/ms-playwright
+          key: playwright-${{ runner.os }}-${{ hashFiles('**/package-lock.json') }}
+
+      - name: Install Playwright browsers
+        if: steps.playwright-cache.outputs.cache-hit != 'true'
+        run: npx playwright install chromium --with-deps
+
+      - name: Install Playwright deps (cached)
+        if: steps.playwright-cache.outputs.cache-hit == 'true'
+        run: npx playwright install-deps chromium
+
+      - name: Build app
+        run: npm run build
+
+      - name: Run E2E tests
+        run: npm run test:e2e
+        env:
+          CI: true
+
+      - name: Upload test results
+        uses: actions/upload-artifact@v4
+        if: always()
+        with:
+          name: playwright-report
+          path: playwright-report/
+          retention-days: 7