Add Integration Tests

[Technical-1]

Priority: Low
Labels: testing, code-quality

Description:
Add integration tests for full application lifecycle. Test UI update flow with mock data and background thread communication.

Files to create:

• tests/integration_test.rs

Acceptance criteria:

• Integration tests for application lifecycle
• Tests for UI update flow
• Tests for background thread communication
• Mock data utilities created
• Tests run in CI/CD pipeline

Implementation notes:

• Create integration test file
• Test full application initialization
• Test refresh cycle with mock data
• Test thread communication and synchronization
• Consider using eframe test utilities if available