Remove redundant createTargetNodeId and use selectedTreeNodeId for both
visual highlight and creation target. This simplifies the state management
by having a single source of truth for tree selection, similar to VSCode's
file explorer behavior where both files and folders can be selected.
Consolidate menu components by extending NodeMenu to support a 'root'
type, eliminating the redundant BlankAreaMenu component. This reduces
code duplication and simplifies the context menu logic by storing
isFolder in the context menu state instead of re-querying tree data.
Add right-click context menu for file tree blank area with New File,
New Folder, and Upload Files options. Also align search input and
add button styles to match Figma design specs (24px height, 6px radius).
