feat: copy files and folders #13

Summary

Copy files and folders to a destination using the operation engine.

Motivation

Copy is a core file-manager operation and does not exist yet.

Requirements

• Writes to a controlled temporary file and finalizes only on success; the source is never modified.
• Recursive folder copy without following junctions/symlinks by default.
• Progress with bytes when measurable; indeterminate progress otherwise.
• Destination chosen through the existing in-app folder picker (or via Paste).

Controller UX

Mark items with West/X → North → Copy to… → pick the folder with the in-app picker → confirm.

Technical Notes

Implemented on the operation engine; the destination picker is AppController.OpenFolderPicker.

Acceptance Criteria

• Copied files are byte-identical; timestamps preserved where possible.
• Cancelled copies leave no partial files under the final names.
• Copying a folder into itself is refused.

Testing

Unit tests for new branches and for every bug reproduced before a fix (no trivial variations, see AGENTS.md). Windows integration tests for long paths and locked files.

Dependencies

• Depends on feat: add a shared file operation engine (plan, progress, per-item results) #10 — feat: add a shared file operation engine (plan, progress, per-item results)

Priority: critical · Milestone: 1 · Core File Operations