fix: Dock panel scrollbars are illogical

[GordonSmith]

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:
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Testing:

[Copilot]

Pull Request Overview

This PR updates the placeholder styling in the DockPanel to prevent unnecessary scrollbars from appearing.

• Changed placeholder overflow style from auto to hidden
• Ensures scrollbars no longer appear when content fits within the panel

Comments suppressed due to low confidence (1)

packages/phosphor/src/DockPanel.ts:91

• [nitpick] Consider adding a brief comment explaining why the overflow is set to 'hidden' here, so future maintainers understand the rationale for removing scrollbars.

.style("overflow", "hidden")