fuzzer web interface: put the source view into a scrolling box so the main UI doesn't go offscreen

[andrewrk]

Extracted from #20958.

All the UI elements on the page should use position: absolute and then set the bottom and right properties. The main window should not scroll, so that all the UI widgets are visible at the same time.

Related:

• fuzzer web interface: add a UI element showing a list of source files sorted by total uncovered points of interest #20983

[geon]

I would suggest the opposite: Let the main window scroll, but position UI elements with position: fixed; to make them stay on screen.

I find that aligns better with web dev and accessibility.