Refactor: Threading architecture with explicit state-machine latching

[flavio-openclaw]

Context

PR #21 implements button handling with background threads, but the architecture has room for improvement.

Problem

1. Threads are spawned inside object constructors
2. Button events are checked in TWO places
3. Implicit synchronization with io_lock
4. No explicit latching mechanism

Solution

Refactor to explicit state-machine pattern with global button flag and mutex.

Related

PR #21: Button/Display implementation