refactor(timer): use CodeMirror Facet for configuration passing

Replace global variable configuration with CodeMirror 6's recommended Facet pattern
for passing configuration to StateField. This provides better type safety,
reactive updates, and follows CodeMirror architecture best practices.

Changes:
- Replace globalTimerConfig with taskTimerConfigFacet
- Update all config access to use state.facet() pattern
- Return array of [facet, statefield] from extension function
- Maintain compatibility with existing functionality

Author: Quorafind