feat: add an optional Pomodoro timer

[CatStack-pixe]

Upstream source

• BongoCat PR #975 by @oliver7l
• BongoCat issue #996 by @xm95270

Port assessment

Port the timer as an optional desktop-pet feature, using MochiPaw's existing stores, preferences, and locale conventions.

Acceptance criteria

• Start, pause, reset, and completion notification work across app restart.
• Timer settings and UI text are localized.
• The feature does not interfere with normal input handling.

[CatStack-pixe]

Closing as completed. The optional Pomodoro timer was implemented and shipped through the merged Pomodoro work in PR #92, including persistence, controls, notifications, localization, and regression coverage.