Drag-and-drop files into the project

> Lower-priority / nice-to-have.

A tester wants to drop external files into the project -- e.g. a figure from a published paper -- so they can compare it against Orbit's analysis output. Today there's no drag-and-drop affordance to get a file into the project.

Related to #92 (side-by-side / compare view in the File tab); this is the input side of the same comparison workflow.

Start in the File tab and trace how files currently enter a project; read related issue #92 for the comparison workflow. Define how an externally dropped file becomes available in the project, then verify it can be compared with Orbit’s analysis output.

Written by the indexing model from the issue text.