Fix same-pane file moves and contextual transfer controls

[bifrost0x]

Summary

• Enable drag-and-drop moves into folders within the same File Manager pane and the embedded Workspace SFTP browser.
• Replace the duplicate directional controls with one contextual inter-pane Move/Transfer button that is shown only for a valid selection and operation.
• Keep embedded Workspace SFTP free of a Move toolbar action or target-browser workflow, and document the drag behavior.

This implements the remaining file-management UI feedback from Discussion #99.

Testing

• JavaScript test suite: 33 passed
• Focused Python UI/template suite: 38 passed
• Targeted Playwright file-workspace and embedded Workspace SFTP scenarios: 3 passed
• ESLint on the changed JavaScript and E2E files
• git diff --check