feat: align timer API contract

[sionsmith]

Summary

• add calendar and desktop time-entry source enum values
• add replace_running plus start_timer, stop_timer, and restart_timer helpers for sync and async clients
• validate started_time and ended_time as HH:mm workspace-time strings
• update README and targeted sync/async tests

Verification

• .venv/bin/pytest tests/test_time_entries.py tests/test_async_resources.py
• .venv/bin/ruff check src/keito/resources/time_entries.py src/keito/types/time_entry.py tests/test_time_entries.py tests/test_async_resources.py
• .venv/bin/mypy src/keito/resources/time_entries.py src/keito/types/time_entry.py
• .venv/bin/pytest