widgets/desktop add calendar as a desktop widget

[Xlntaro]

Summary

• Adds a built-in calendar widget to the Desktop Widgets Editor.
  • Supports month navigation, mouse-wheel scrolling, selecting days, and returning to the current date.
  • Displays event indicators for each day and an optional event list for the selected date.
  • Reuses the existing CalendarService, so connected Google Calendar and CalDAV events are displayed automatically.
  • Adds settings for showing events, displaying ISO week numbers, selecting a font, and configuring the standard widget background.
  • Adds removable calendar change subscriptions so editor previews can be safely recreated.
  • Adds configuration validation coverage for the new widget type.

Motivation

The calendar was previously available only inside the Control Center. This adds a persistent desktop version with the same calendar data and day-selection behavior

Type of Change

• Bug fix
• New feature
• Breaking change
• Refactoring
• Build / packaging

Related Issue

Testing

Manual Coverage

• Tested on Niri
• Tested on Hyprland
• Tested on Sway
• Tested on another compositor:
• Tested with different bar positions and density settings
• Tested at different interface scaling values
• Tested with multiple monitors

Screenshots / Videos

Checklist

• This PR is ready for review, or it is marked as Draft.
• I read and followed the relevant guidance in CONTRIBUTING.md.
• I ran just format with clang-format v22+ installed, or this PR has no code changes.
• I ran the relevant build or test commands, or explained why they were not run.
• I self-reviewed the changes.
• I checked for new warnings or errors.
• I will update end-user documentation after merge, or this PR does not change user-facing configuration or behavior.
• I added or updated assets/translations/en.json, or this PR adds no new user-facing strings.
• I did not edit non-English translation files unless this PR is explicitly for translation tooling, an import/export sync, or a maintainer-requested locale change.
• I used the existing canonical names for config keys, IPC names, paths, and identifiers.

Additional Notes

[Reiling-Jeff]

Thats so cool