Add drag-and-drop loading for .cfm.json files

[suspiriaa]

Summary

Allow users to drag a .cfm.json file onto the app window to load a recording.

Motivation

This would make the desktop workflow faster and reduce friction for common load operations.

Scope

• detect supported file drops on the main window
• validate that the file extension is supported
• reuse the existing serializer and load validation flow
• show a clear error if the file is invalid

Acceptance Criteria

• dragging a valid .cfm.json file into the app loads the recording
• invalid files show a user-friendly error
• the existing menu/button based load flow keeps working