feat(tui): add light theme support with auto-detection

[Ayan-sh03]

Fixes #63

Changes

• Support SHAI_TUI_THEME environment variable (dark/light) for auto-detection on startup
• Added /theme command with dark, light, and toggle options

How to Use

Set theme via environment variable:

SHAI_TUI_THEME=light shai

or use commands at runtime

/theme light (or dark)

Screenshots:

light theme

dark theme

Signed-off-by: Ayan ayansheikh52@gmail.com

[Marlinski]

THanks for your contribution I am merging this one!

Would be nice to have another PR for automatic background detection and theme selection.
Looking around it seems it can be achieved with special ["\e]11;?\a"](https://stackoverflow.com/questions/2507337/how-to-determine-a-terminals-background-color) there's even a small library in rust though not sure how easy it would be to use it.