Layout Tray that is collapsed, but expands on hover

[davidmh72]

Context

This PR modifies the Grist layout to improve workspace efficiency and visual clarity by customizing the behavior of the Layout Tray. The default Layout Tray permanently occupies space even when empty or unused, reducing the effective working area.

Proposed solution

• In LayoutTray.ts, the tray was restructured to behave as an overlaid floating element:

  • It remains hidden when empty.
  • When it contains widgets, it collapses into a minimal strip.
  • A green line serves as a visual indicator that widgets are present.
  • On hover, the tray expands over the main content rather than pushing it down.

• In GristDoc.ts, the layout padding in cssViewContentPane was reduced by modifying --view-content-page-padding to eliminate the unnecessary spacing above the tray when collapsed.

These changes are fully isolated and marked with // MOD DMH comments for maintainability.

Related issues

This customization is part of a UI refinement effort. No related issues are currently filed.

Has this been tested?

• 🙅 no, because this is covered by manual verification in a production-like environment

Screenshots / Screencasts

Not included. Visual confirmation has been performed manually.