feat: align timer API contract

[sionsmith]

Summary

• update OpenAPI/generated types for timer sources, replace_running, /stop, and /restart
• add startTimer, stopTimer, and restartTimer helpers for server-owned running timers
• preserve API error bodies on KeitoConflictError so callers can inspect running_entry for running_timer_conflict
• update README and targeted tests

Verification

• npm run generate
• npm run test -- tests/resources/time-entries.test.ts tests/core/http.test.ts
• npm run typecheck
• npm run test