Calendar plugin: support event colors stored in event notes

[michalkrause]

Enable support for custom event colors saved within notes, providing compatibility with 'Calendars by Readdle' and 'Calendar 366' apps.

Description

This PR adds support for parsing event colors stored inside the event notes. This behavior is used by some third-party calendar applications like 'Calendars by Readdle' and 'Calendar 366' to persist custom colors.

Motivation

Users of these alternative calendar apps currently lose their color-coding when events are displayed in the Calendar plugin. This change improves ecosystem compatibility for those users.

Risk Assessment & Implementation

While this addresses a non-standard feature specific to certain apps, the implementation is very straightforward and isolated:

• It safely falls back to default behavior if no color data is found in the note.
• It introduces no breaking changes or performance overhead for standard events.

[jgclark]

Thank you Michael. I will update the documentation to cover this and then release.

[michalkrause]

Thanks for accepting the patch!