[Linux] - Fix broken header images.

[ZereoX]

I found that the new assets from #9063 weren't being loaded when using PoBFrontend. This PR simply updates the filename case to match the images in the Assets folder. This specifically updates the Separators, Headers, and Influence Icons.

Description of the problem being solved:

Starting with #9063, assets for Headers (Left, Right, Middle, Icons) and Separators aren't being picked up by Path of Building when using PoBFrontend due to case-sensitivity.

Steps taken to verify a working solution:

• This was only tested locally by applying the MR on top of the existing AUR build (https://aur.archlinux.org/packages/path-of-building-community-git)
  • ZereoX/pobfrontend@af7f1af is also required to support the new FONTIN fonts.
• I confirmed the following were rendering correctly with the PR applied:
  • Items (Normal, Magic, Rare, Unique, Relic, All Influences, Jewels)
  • Nodes (Keystone, Mastery, Notables, Normals).

Before screenshot:

Abberath's Hooves	Veteran's Wrath

After screenshot:

Abberath's Hooves	Veteran's Wrath

[Wires77]

I actually think I'd prefer changing the filenames instead of this. Just checking with the original author of this code to make sure there wasn't a reason to do one way or the other.