feat: Fixes issue #498 ✨ add DividerSettings for customizable dividers in WeekView and MultiDayView

[lavigarg-simform]

Description

This PR adds customizable divider settings for the divider line between the weekdays header and full-day events section in both WeekView and MultiDayView.

Core Feature Implementation:

• Introduced a new DividerSettings class with three configurable properties:
  • thickness: Controls divider line thickness (default: 1.0)
  • height: Controls divider height (default: 1.0)
  • color: Controls divider color (default: Colors.grey)
• Added a dividerSettings parameter to both WeekView and MultiDayView widgets.
• Applied dividerSettings in internal WeekView and MultiDayView page rendering.
• When dividerSettings is not provided, both views fall back to theme border color via context.multiDayViewTheme.borderColor.
• Added DividerSettings.none() factory constructor for an invisible divider.
• Replaces previously fixed divider styling with configurable behavior.

Documentation:

• Added divider customization guidance and usage examples in docs, including:
  • Custom DividerSettings configuration
  • DividerSettings.none() usage to hide the divider

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

Related Issues

Closes #498