Optimize SVG illustration file sizes

[taea]

Related: #3727

I optimized some of the illustration images used on the top page using svgo.

Some illustration files were already optimized, but there were a few files where optimization caused rendering issues in certain browsers, so I had temporarily decided not to optimize them.
This time, I attempted to optimize and reduce the file sizes of those files as well.

While some files didn't change much, the total reduction was -29.1 KiB, approximately 14.6%.

Details

File	Before	After	Reduction
boxes.svg	3.527 KiB	3.484 KiB	-1.2%
sunburst.svg	9.948 KiB	5.155 KiB	-48.2%
illust-sub.svg	28.563 KiB	26.254 KiB	-8.1%
coffee.svg	3.995 KiB	3.968 KiB	-0.7%
fire.svg	16.809 KiB	13.306 KiB	-20.8%
ivan.svg	11.163 KiB	9.168 KiB	-17.9%
justin.svg	7.502 KiB	7.494 KiB	-0.1%
keyboard.svg	5.317 KiB	5.29 KiB	-0.5%
mimi.svg	6.005 KiB	5.119 KiB	-14.8%
robot.svg	4.331 KiB	4.329 KiB	-0%
train.svg	56.338 KiB	49.303 KiB	-12.5%
bg.svg	4.842 KiB	2.58 KiB	-46.7%
illust-pattern.svg	41.058 KiB	34.849 KiB	-15.1%

Total: 199.4 KiB → 170.3 KiB (-29.1 KiB, approximately 14.6% reduction)

Tested Environment

Tested on my development environment (macOS 15.6) with the following browsers:

• Chrome 143.0.7499.110
• Safari 18.6
• Firefox 146.0

[JuanitoFatas]

😍