feat(timer): add on_finished/on_cancelled listeners and time_remaining property

[Faerkeren]

Summary

• Add on_finished and on_cancelled listener decorators that distinguish natural timer expiry from explicit cancellation, using HA dedicated timer.finished and timer.cancelled event types
• Add time_remaining computed property that returns live seconds remaining -- calculated from finishes_at when active, parsed from remaining attribute when paused
• Subscribe to timer.finished and timer.cancelled events at the client level and route to Timer entities
• 10 new tests covering all new functionality (event listeners, computed property edge cases)
• All 156 tests pass, 95.22% coverage, lint/mypy clean