refactor: replace custom integration tests with working TasksUITest from PR #123

**Removed Cruft:**
- ❌ SimpleIntegrationTest.kt (complex Ditto sync test with seeding logic)
- ❌ TasksSyncIntegrationTest.kt.disabled (unused disabled test)
- ❌ Document seeding and environment variable dependencies
- ❌ Complex retry logic and cloud sync verification

**Added Clean Test:**
- ✅ TasksUITest.kt (proven working test from PR #123)
- ✅ Simple UI interaction testing (add task flow)
- ✅ Memory leak detection test
- ✅ No external dependencies or seeding required

This matches exactly what the working PR uses and removes all the
experimental integration test complexity we built up.

Author: teodorciuraru