feat: merge RFC 7986 COLOR property for calendar components

[SashankBhamidi]

Closes #17

RFC 7986 §5.9 defines COLOR as a CSS3 color name that can appear on VCALENDAR, VEVENT, VTODO, and VJOURNAL. Apple extends this with X-APPLE-CALENDAR-COLOR at the calendar level.

This propagates the source calendar's resolved color to any component that lacks its own COLOR. When merging multiple calendars, the first calendar with a color wins for the merged VCALENDAR top-level property. Components that already declare COLOR are left untouched.

Bumps icalendar to >=7.0.0 to use the typed .color API (requires Python 3.10+). Also fixes latent UID-less deduplication bugs for VTODO and VJOURNAL components.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/mergecal/calendar_merger.py	100.00% <100.00%> (ø)
src/mergecal/cli.py	47.82% <ø> (-2.18%)	⬇️

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[niccokunzmann]

Some changes :)

[niccokunzmann]

Just a little change and that is it.
Documenting intention and not code.