fix: increase the resize handle hit zone

[GordonSmith]

Increased from 1px to 4px

Checklist:

• The commit message is properly formatted and free of typos.
  • The commit message title makes sense in a changelog, by itself.
  • The commit message includes a "fixes" reference if appropriate.
  • The commit is signed.
• The change has been fully tested:
  • I have viewed all related gallery items
  • I have viewed all related dermatology items
• There are no similar instances of the same problem that should be addressed
  • I have addressed them here
  • I have raised new issues to address them separately

Testing:

[Copilot]

Pull request overview

Adjusts dgrid’s column resize UX by enlarging the interactive resize handle area, making column resizing easier to grab.

Changes:

• Increased the .dgrid-resize-handle transparent border size to widen the resize hit target.
• Updated packages/dgrid/index.html to include two table examples (nested columns + resize-handle-focused example) and generalized placeholder styling via a CSS class.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
packages/dgrid/src/Common.css	Enlarges the resize handle hit zone via a wider transparent border.
packages/dgrid/index.html	Updates the local test page to exercise nested tables and resizing behavior, and reworks placeholder styling.