Add comprehensive integration test suite for update flow

Implements end-to-end testing infrastructure for the Pulse update flow,
validating the entire path from UI to backend with controllable test
scenarios.

## What's Included

### Test Infrastructure
- Mock GitHub release server (Go) with controllable failure modes
- Docker Compose test environment (isolated services)
- Playwright test framework with TypeScript
- 60+ test cases across 6 test suites
- Helper library with 20+ reusable test utilities

### Test Scenarios
1. Happy Path (8 tests)
   - Valid checksums, successful update flow
   - Modal appears exactly once
   - Complete end-to-end validation

2. Bad Checksums (8 tests)
   - Server rejects invalid checksums
   - Error shown ONCE (not twice) - fixes v4.28.0 issue type
   - User-friendly error messages

3. Rate Limiting (9 tests)
   - Multiple rapid requests throttled gracefully
   - Proper rate limit headers
   - Clear error messages

4. Network Failure (10 tests)
   - Exponential backoff retry logic
   - Timeout handling
   - Graceful degradation

5. Stale Release (10 tests)
   - Backend refuses flagged releases
   - Informative error messages
   - Proper rejection logging

6. Frontend Validation (15 tests)
   - UpdateProgressModal appears exactly once
   - No duplicate modals on error
   - User-friendly error messages
   - Proper accessibility attributes

### CI/CD Integration
- GitHub Actions workflow (.github/workflows/test-updates.yml)
- Runs on PRs touching update-related code
- Separate test runs for each scenario
- Regression test to verify v4.28.0 issue prevention
- Automatic artifact uploads

### Documentation
- README.md: Architecture and overview
- QUICK_START.md: Getting started guide
- IMPLEMENTATION_SUMMARY.md: Complete implementation details
- Helper scripts for setup and test execution

## Success Criteria Met

✅ Tests run in CI on every PR touching update code
✅ All scenarios pass reliably
✅ Tests catch v4.28.0 checksum issue type automatically
✅ Frontend UX regressions are blocked

## Usage

```bash
cd tests/integration
./scripts/setup.sh    # One-time setup
npm test              # Run all tests
```

See QUICK_START.md for detailed instructions.

Addresses requirements from issue for comprehensive update flow testing
with specific focus on preventing duplicate error modals and ensuring
checksum validation works correctly.

tests/integration/mock-github-server/Dockerfile

@@ -0,0 +1,18 @@
+FROM golang:1.23-alpine AS builder
+
+WORKDIR /build
+COPY go.mod ./
+RUN go mod download
+
+COPY main.go ./
+RUN CGO_ENABLED=0 GOOS=linux go build -o mock-github-server .
+
+FROM alpine:latest
+RUN apk --no-cache add ca-certificates
+
+WORKDIR /app
+COPY --from=builder /build/mock-github-server .
+
+EXPOSE 8080
+
+CMD ["./mock-github-server"]