feat(tui): highlight hex codes with the color

[jettptacek]

What does this PR do?

Highlights hex color codes in markdown content with their actual color as the background. The foreground text automatically switches between black and white based on luminance for readability. Supports:

• 3-char (#rgb)
• 4-char (#rgba)
• 6-char (#rrggbb)
• 8-char with alpha (#rrggbbaa) Colors with alpha are blended against the theme background, and update correctly when switching themes.

Closes #2586

How did you verify your code works?

Manual verification in Wezterm and Powershell on Windows

https://github.com/user-attachments/assets/dfc4aac9-1d11-492c-bfe8-ddb8776ee3e9