feat: add copy path context menu option to file tree

[ianphil]

Closes #46

Adds a right-click context menu to the file tree with a Copy Path option. Works for both files and folders.

Changes

• FileTreeNode: Added optional onContextMenu prop that fires on right-click, passing the node's path upward
• FileTree: Integrated useContextMenu hook to manage context menu state, renders a context menu overlay with a Copy Path button that writes the path to the clipboard via navigator.clipboard.writeText()
• Icon: Added copy to the IconName union (uses VS Code codicon codicon-copy)
• Tests: Added 2 new tests covering right-click copy path for files and folders

All 33 existing tests continue to pass.