Add an always-accessible drop zone, instead of Reset & Upload New

First of all, **thank you** for making such an awesome tool. I use this every day, and it has motivated me to use JMH more, leading to better understanding of the performance of my code.

My current workflow:

1. View benchmark results.
2. Update the code (or the benchmark) and run it again.
3. Menu > Reset & Upload New
4. Drag new JSON files into the drop zone.
5. Repeat.

I think step 3 could be eliminated if there was an always-accessible drop zone. In other words, allow JSON files to be dropped onto the JMH visualizer while it is displaying results.

Start by locating the existing “Reset & Upload New” menu action and the current JSON drop zone in the visualizer. Trace how dropped files replace displayed results, then verify that dropping JSON files while results are visible loads the new results without requiring a reset.

Written by the indexing model from the issue text.