fix(tui): preserve workspace preview across child rails

[lawrencecchen]

Summary

Keep a workspace preview active while focus moves from the workspace rail into the tabs or projection (agents) rail. Those child rails now continue to show the highlighted workspace. Moving to the machine rail or pane still cancels the preview.

Testing

• rustfmt --edition 2024 --check cmux-tui/crates/cmux-tui/src/app.rs
• git diff --check
• Hosted focused Linux and macOS verification: pending
• Regression test: workspace_preview_survives_workspace_dependent_rail_navigation

Review Trigger

Stacked on #10736.

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Summary by cubic

Keeps the workspace preview active as focus moves between the workspace, tabs, and projection rails, so those child views keep following the highlighted workspace. Activating a tabs rail target now commits the preview; moving to the machine rail or pane still cancels it and restores the original active workspace.

• Adds regression tests for previews preserved across child rails, hover previews committed on row click, and previews committed when a tabs rail target is activated.

^{Written for commit e062c7f. Summary will update on new commits.}

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[lawrencecchen]

Superseded by PR #10969, which includes this child-rail persistence work in the unified preview stack. Closing this intermediate stack member to avoid duplicate app.rs merges.