feat(fullscreen): add Duplicate option to file context menu

[KrisPowers]

Summary

• Adds a Duplicate entry to the file explorer right-click menu (files only, not folders)
• Generates the copy name as <name> copy.ext, incrementing to <name> copy 2.ext, <name> copy 3.ext, etc. if the name is already taken
• After the copy completes, the rename input opens automatically on the new file so the user can rename it immediately
• Adds fs.copy IPC op to the C++ backend (std::filesystem::copy with recursive and copy_symlinks options), with a matching declaration in fileops.hpp

Test plan

• Right-click a file in the sidebar and confirm Duplicate appears in the menu
• Clicking Duplicate creates <name> copy.ext in the same folder and opens the inline rename input
• Duplicating when <name> copy.ext already exists produces <name> copy 2.ext, and so on
• Files without an extension (Makefile, Dockerfile) get <name> copy as the copy name
• Pressing Escape in the rename input cancels the rename but leaves the copied file in place
• Directories do not show the Duplicate option

Closes #75