fix: Prevent black flash when switching workspaces to split view

[moaead]

Summary

Fixes #11965

Problem

When switching from a regular workspace to a workspace containing a split view, there is a brief black flash around the split view area. This flash does not occur when switching between two workspaces that both have split views.

Solution

The black flash was caused by a timing issue in how CSS attributes are applied during split view activation. The CSS rules hide containers that have is-zen-split but not zen-split attribute when the parent has zen-split-view attribute.

The fix reorders the attribute setting in activateSplitView():

1. Call applyGridToTabs() first to set zen-split attribute on containers
2. Then set zen-split-view attribute on parent elements

This ensures containers have their zen-split attribute set before the parent zen-split-view CSS rules take effect, preventing the visibility flash.

Changes

• src/zen/split-view/ZenViewSplitter.mjs: Reordered applyGridToTabs() call in activateSplitView() to run before setting parent attributes, and added zen-split attribute setting in applyGridToTabs() for consistency

Testing

• Create a split view in workspace A
• Switch to workspace B (without split view)
• Switch back to workspace A (with split view)
• Verify no black flash occurs around the split view area
• Verify switching between two workspaces with split views still works correctly

Checklist

• Code follows project style guidelines
• Changes are minimal and focused
• Linked to issue with 'Fixes #'