fix: Address #12631 with activeModalWidget() checks

[Jesssullivan]

Hi there!

This is a pass at addressing #12631; I bumped into this as well.

I just added activeModalWidget() checks in src/gui/DatabaseWidget.cpp; AFAICT this is all that is needed, though I am new to this codebase.

Testing strategy

• added a test to verify activeModalWidget()'s intended blocking behavior in TestGui; noting I am unfamiliar with this test setup.
• Ran through the tests locally on 26.2 (25C56), apple silicon (m3). Would appreciate some pointers on how others are running local multiarch tests.

Type of change

• ✅ Bug fix