feat: add server browser map previews

[XJle6uIIIeK]

Adds generated map previews to the server browser, shown after hovering a server entry.

What changed

• Adds a background map preview manager with download, generation, cache loading and prioritization.
• Prioritizes the map currently being hovered while continuing background processing for other maps.
• Uses map URL, SHA-256 and size metadata from the server list when available.
• Supports DDNet SHA-256 downloaded-map filenames and community map URLs.
• Generates previews directly from downloaded map files.
• Renders original embedded/external map textures instead of a placeholder representation.
• Preserves map background/parallax zoom when generating the preview.
• Caches generated PNG previews so subsequent hovers do not regenerate the map.
• Shows preview/status UI in the server browser without changing gameplay or map physics.

Validation

• git diff --check passes.
• The same generated source revision was built successfully as the Windows x64 game-client in CI before moving it into this fork.
• The final PR branch is based directly on BestProjectTeam/BestClient@909b4aafe1b4a82e5e559e06ea419b9c9fa53479.

Checklist

• Tested the change ingame
• Provided screenshots if it is a visual change
• Tested in combination with possibly related configuration options
• Written a unit test (especially base/) or added coverage to integration test
• Considered possible null pointers and out of bounds array indexing
• Changed no physics that affect existing maps
• Tested the change with ASan+UBSan or valgrind's memcheck (optional)
• I didn't use generative AI to generate more than single-line completions

[XJle6uIIIeK]

[RoflikBEST]

yo, I've been wanting to implement such a feature for a long time, thank you

[RoflikBEST]

I will accept the request and then reverse it, because I am currently working on 3.0, I will transfer the implementation there.