fix file drops when standalone page uses file://

[wgnrto]

Summary

Fix setDropState() and EventListener in wireUp().

Problem

Drag-and-drop of the source file into the dedicated area in browser/mechcheck.html. Did not work and throws the following errors:

Uncaught TypeError: Cannot read properties of null (reading 'addEventListener')
    at wireUp (mechcheck.html:3695:17)
    at mechcheck.html:3718:1
mechcheck.html:3693 Uncaught (in promise) EncodingError: A URI supplied to the API was malformed, or the resulting Data URL has exceeded the URL length limitations for Data URLs.
mechcheck.html:1 Unsafe attempt to load URL file:///mechcheck-main/mechcheck-main/browser/mechcheck.html from frame with URL file:///mechcheck-main/mechcheck-main/browser/mechcheck.html. 'file:' URLs are treated as unique security origins.

Testing

• Opened browser/mechcheck.html by double-clicking it.
• Dropped an Overleaf source ZIP.
• Confirmed that the project was read successfully.