Add unit tests for src/app.rs

[ih8js-git]

Why it needs tests: This file contains the core state machine of your application (handle_key_event transitions).

How to test it: Currently, App is heavily coupled with I/O (it instantiates Auth and CacheStore directly). To easily unit test the state transitions, you might want to either mock those dependencies or extract the pure state-transition logic out of the side-effect-heavy App struct.