feat: unify calendar picker in event/task editor

[naveensingh]

Type of change(s)

• Bug fix
• Feature / enhancement
• Infrastructure / tooling (CI, build, deps, tests)
• Documentation

What changed and why

• Unified the calendar picker in the event editor and made relevant changes related to calendar management. This does not display the "Add a new calendar" option that was previously available in the local calendar picker, but it still has the Manage calendars option for those who need it.
• Added an explicit "Offline (never synced)" label when a local calendar is selected
• Updated calendar picker icon in task editor

Tests performed

• Testing switching between calendars in the event editor

Closes the following issue(s)

• Closes Replace event types concept with calendars #629

Checklist

• I read the contribution guidelines.
• I manually tested my changes on device/emulator (if applicable).
• I updated the "Unreleased" section in CHANGELOG.md (if applicable).
• I have self-reviewed my pull request (no typos, formatting errors, etc.).
• All checks are passing.