Fix alphanumeric sorting + various design tweaks

I made separate commits:

• Fix tooltip overflowing body

When you hover over an entry in the file browser until the popup appears, and then scroll quickly up until the tooltip leaves the screen, a scrollbar appears on the body which shifts the layout. The current overflow: hidden fix was on the wrong element. I moved it to the body.

• Remove .h5.dataset extensions in file browser entries

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• Fix alphanumeric sorting algorithm (reported by Romain Talon)

The previous sorting was rudimentary, putting all upper case letters first and 10 before 2. I've switched to using Intl.Collator, which has excellent browser support.

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• Add spaces in vis title around the separator between the HDF5 container and the dataset name

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• Tweak file browser styles and extract error fallback

There was a bit too much horizontal padding in the file browser for my taste, including in the error fallback. While I was at it, I created the DirectoryErrorFallback component to streamline FileBrowser, and extracted the ErrorFallback styles into a CSS module.

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