feat: add dual-pane file management #56

Summary

Two independent panes with copy/move/extract to the other pane.

Motivation

Classic two-panel workflow for organizing files; required by the spec (journey C).

Requirements

• Each pane keeps location, history, selection and scroll; the active pane is unmistakable.
• Actions: Copy to other pane, Move to other pane, Extract to other pane (from an archive pane) — showing source and destination before running.
• Switching panes never changes selection nor starts an operation.

Controller UX

LB/RB switches the active pane; North → Copy/Move/Extract to other pane.

Technical Notes

Second PaneState in AppController; operations via the engine and IArchiveService.

Acceptance Criteria

• Copy/move/extract to the other pane show source and destination and complete correctly.

Testing

Unit tests for new branches and for every bug reproduced before a fix (no trivial variations, see AGENTS.md). Journey test for journey C of the spec.

Dependencies

• Depends on feat: copy files and folders #13 — feat: copy files and folders
• Depends on feat: move files and folders (same-volume rename, cross-volume copy then delete) #14 — feat: move files and folders (same-volume rename, cross-volume copy then delete)

Priority: medium · Milestone: 4 · Navigation & Search