feat: add dark mode for `wrtagweb`

[coxde]

Hello again! While script automation works great, sometimes I have to use the web ui to manually import or just checking, it might be a good idea to have a dark mode for my eyes :xD

Based on the complexity, I break it down into:

• Have a toggle on the web page
• Use browser/system variables to set light/dark mode
• Have a drop down menu to choose light/dark/auto mode

Any thoughts?

I did make a demo using vibe coding, but I can take a deeper look since I’m not familiar with go template html. Also is there a better way to test it rather than building the whole container like I did? Thanks for reading!