[FEAT] Auto-detect color theme with the terminal

Motivation

I have color scheme of the terminal synced to the OS theme, so Claude Code looks like this with the light terminal background

<img width="331" alt="Image" src="https://github.com/user-attachments/assets/a8e7271f-7163-4b87-a855-34020ec67143" />

unless I manually change it to light theme with /theme.

Proposed solution

On startup, or ideally once in a while, check the color scheme of the terminal, and choose dark or light theme depending on it.

Use terminal escape codes OSC 10 / OSC 11 to detect terminal color scheme. See https://github.com/sharkdp/bat/pull/2896 for inspiration.