feat: Add dark mode toggle in Settings

[caseyg]

• Removed theme toggle from the main toolbar
• Added dark mode toggle in Settings > General under Language
• Set up theme synchronization across windows using Tauri events
• Fixed border styles and visual elements in dark mode
• Improved toast notifications in dark mode
• Modified Tauri window creation to follow system theme preferences

🤖 Generated with Claude Code

There might be a less verbose way to do this than so many inline Tailwind classes, but I wanted to see if it was possible to do it quickly with Claude Code. Looks good (enough!) to me on the frontend.

Also tried to get it to sync with system/user preference but wasn't able to figure out how to get that to work with Tauri quickly, so for now just implemented as a toggle in Settings > General.

[yujonglee]

[ComputelessComputer]

Cool! I guess If you're done with implementing the details, we could start from that. Thanks @caseyg :)

[caseyg]

sure thing! feel free to use whatever part of this is helpful 👍