[Bug]: Event creation: cover image + date/time

[RishiAnand108]

Description

Improve the event creation flow when creating an event from a group.

Changes Required

1. Cover Image Upload

   • Currently, event creation asks for a cover image via URL.
   • Add an option to upload a cover image directly.
   • Users should be able to choose an image from their device instead of being required to provide a URL.

2. Date and Time Selection

   • Borrow the date/time picker design and interaction used in Blog Schedule.

   • Use the same or a consistent UI for selecting:

     • Event date
     • Event start time
     • Event end time, if supported

Acceptance Criteria

• Users can upload an event cover image directly.
• URL-based image input does not unnecessarily replace the upload option.
• Event date uses the Blog Schedule-style date picker.
• Event time uses the Blog Schedule-style time picker.
• Date and time selection is clear and consistent with the existing product UI.
• Existing event creation functionality continues to work.

[linear-code]

THE-119