feat: optimise and update gemini icons

[piemot]

Changes proposed:

• Add
• Delete
• Fix
• Prepare

Builds on #3891.

Updates the Gemini file icon:

• Removes transparency
• Removes usage of poorly-supported SVG features
• Reduces file size
  

Update the Gemini folder icons:

• Removes transparency
• Updates folder colour
• Reduces file size

[github-actions]

Icons Background Test

file_type_gemini (4569 bytes)

folder_type_gemini (4985 bytes)

folder_type_gemini_opened (5024 bytes)

[piemot]

There's a few more opportunities for file-size optimisation (the most obvious being that there's a duplicate path that could be extracted into a <use /> block), but doing that made Inkscape refuse to import it properly so I figured half a megabyte was worth the portability.