Drag and drop a .yml onto the window to open it

[rebaserHEAD]

No drop handling exists anywhere today. Dropping a map/grid .yml onto the editor should import it, same confirm-if-dirty flow as File > Import.

• Works in both builds: dataTransfer.files gives content everywhere; on desktop the File path is also available so the file can be recorded in recent files with its fork context.
• Drop on the start screen could do the full fork-then-file resolve if the file lives under a known fork; fine to scope that part out of v1.