Clear file tree selection on pane click

[jsgrrchg]

Summary

This PR clears explicit multi-selection in the file tree when the user clicks back into an editor/workspace pane.

Why

Multi-selected files and folders stayed visually selected after focus moved back to the editor, which made the tree feel like it had stale contextual state. The tree should keep multi-selection while the user is operating inside it, but clear that contextual selection when another main pane becomes active.

Changes

• Adds a shared clearFileTreeSelection UI event in the navigation utilities.
• Emits that event from workspace panes on pointer down, alongside the existing pane focus behavior.
• Keeps the file tree selection state encapsulated inside FileTree by having it listen for the cleanup event.
• Adds focused tests for both the workspace event emission and file tree cleanup behavior.

Validation

• npm test -- --run src/features/vault/FileTree.test.tsx src/features/editor/MultiPaneWorkspace.test.tsx
• npx tsc -b --pretty false