Add color setting for calendar entities

[Misiu]

Breaking change

Proposed change

Add color setting for calendar entities for home-assistant/core#145606, similar to home-assistant/core#88139

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (thank you!)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example configuration

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue or discussion: Add color and icon to the CalendarEntity architecture#883
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

[MindFreeze]

Drafting until the comment and Core PR are resolved

[Misiu]

As mentioned in home-assistant/core#145606, I think that backend changes aren't needed as the color is stored in the entity registry.
Docs PR: home-assistant/developers.home-assistant#2919 that describes color management for calendars.
I've added spinner and color synchronization, so the calendar card updates automatically when, for example, you change color in a second browser.
This change in the frontend is bigger than I expected, and I think there's a better way to do it, but as I'm not a frontend developer, I wasn' table to create something simpler. I've tried using Gemini 3 Pro and Claude Sonnet 4.5 to simplify the changes, but it didn't create anything useful, so feedback is more than welcome

[MindFreeze]

I simplified it a bit.
I think we do need the core changes or the color setting will be rejected by validation.

[Misiu]

Thank you for your time and for the changes.
I've started Core from dev (no changes from my branch), and everything worked fine. Not sure what validation needs to be changed/added.

[Misiu]

I've added color to Google Calendar - home-assistant/core@b085d37. It works just fine with this PR.

The latest changes simplify things a bit; now the frontend requires a hex string, so the color can be correctly seen in entity options

[Misiu]

Backend PR is approved

[Misiu]

@MindFreeze thank you for the review.
This works, but I believe there are a lot of changes in the code.
Maybe there is a better/simpler way to load colors from the entity registry. Especially in hui-calendar-card.ts and ha-panel-calendar.ts
I intended to avoid changing the colors while loading (at first, the color was calculated by index, then changed when values were loaded from the entity registry), but also to update the color on cards as soon as entity options are changed.

Maybe there is an easy way to simplify this code?

[MindFreeze]

I added SubscribeMixin to simplify a bit more. Don't see what more can be done so I'm going to call it good enough. Thanks