Add a showHourLines preference (default off — the existing clean look).
When on, the week and day timelines draw a faint outline-variant line at
each hour boundary, sitting over the column background but beneath event
blocks. The toggle is provided app-wide via LocalShowHourLines and applied
through a reusable hourSeparatorLines() modifier.
Closes #5 (Codeberg)
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
