refactor+feat: guider-panel resolveTargetCoords, disable guide on active, exptime on scicam expose

- Extract resolveTargetCoords() to eliminate center/stick/custom logic duplicated
  in onTargetChange and the non-destructive right-click handler
- Disable Guide button via noctua-telemetry when guider-active is truthy
- Disable all guider exptime inputs via noctua-telemetry when scicam snapshot-state=2 (Exposing);
  onCameraChange no longer unconditionally disables exptime for passive cameras

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>