Fix DevTools shell pane resizing by binding grid tracks to split variables

[ypingmoe]

Summary

This PR fixes a layout synchronization bug in the DevTools shell (packages/react-devtools-shell/index.html) where dragging the split divider updated the divider position visually, but did not reliably resize the iframe and DevTools panes.

The root cause was split state being updated in one place while grid track sizing could diverge. The fix makes grid sizing consistently variable-driven and toggles layout mode via container classes, so both divider behavior and pane dimensions are derived from the same source of truth.

Changes

• Add explicit CSS variables for split sizing (--left-column-width, --top-row-height) and divider thickness.
• Define layout-specific grid templates via #panes.leftright and #panes.topbottom.
• Update setLayout() to:
  • set the relevant split CSS variable
  • switch layout classes on #panes
• Keep divider sizing tied to --divider-size for consistency across orientations.
• Remove manual divider offset coupling that could drift from actual grid track sizing.

Why this approach

Using CSS variables as the single source of truth for grid tracks ensures drag interactions and rendered pane sizes remain in sync. Class-based layout switching also reduces imperative style mutations and makes orientation behavior easier to reason about and maintain.

Testing

• Run DevTools shell locally and open the shell page.
• Verify Left/Right Split:
  • drag divider horizontally
  • both panes resize continuously with pointer movement
• Verify Top/Bottom Split:
  • drag divider vertically
  • both panes resize continuously with pointer movement
• Switch layouts repeatedly and confirm split behavior remains stable.

Fixes #36434

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