chore(assets): compress large city.png mat image

[Animation-EdNum]

Optimized the city.png mat image in assets/img/mats/ using Python's Pillow library. By converting the image to an adaptive 256-color palette (P mode) and saving with optimization enabled, the file size was successfully reduced from 6.9 MB to ~1.3 MB while preserving visual quality and hard lines. This reduces load time and improves repository health.

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PR created automatically by Jules for task 14273414378586147922 started by @Animation-EdNum

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