[BUGFIX] Chart Editor - Variation fallback & chartdata only difficulties in difficulty toolbox

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Linked Issues

Fixes #5206 (For cases when a chartdata difficulty isn't in the metadata)

Description

Fixes the variation fallback - if there's no default variation then it'll use the first available variation.

Difficulties that exist in the chartdata but are missing in metadata for that variation now appear in the difficulty toolbox. They aren't added to the metadata, however.

Screenshots/Videos

2025-06-09.21-28-58.mp4

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One thing of note, since this adds missing chartdata difficulties to the metadata difficulties for that variation, this means that stuff like picospeaker will show up in the difficulties, and it'll be saved as a playable difficulty?

I think instead I'll just make it add the difficulties to the difficulty toolbox instead, and later, I can add a button to add/remove said difficulty from the metadata, and add some kind of visual in the metadata toolbox.

Until that stuff, there'll be a few cases I'm sure of someone complaining why their difficulty shows up in the chart editor but not in freeplay or whatever, because the chart editor does not indicate that the difficulty isn't in the metadata.