Practice and interview

The closing stage of the branch: turning knowledge into working practice. Error handling in async code, writing tests with `package:test` and structuring code so it can be tested, the everyday tool chain of pub, pubspec and the analyzer, a wrong-versus-right catalogue of classic Dart pitfalls, and finally the interview itself — 20-30 second model answers to rapid-fire questions and the live-coding process: clarify, state the approach and complexity, narrate while coding, dry-run, discuss trade-offs.