feat: improve theme selector accessibility and visual feedback

### 🔖 Enhancement description

The current theme selector could provide a better user experience by improving visual feedback and accessibility.

### 🎤 Pitch

Proposed improvements
Add icons for Light, Dark, and System themes.
Highlight the currently selected theme more clearly.
Improve keyboard navigation and screen reader support.
Add descriptive tooltips for each option.

Benefits
Improves accessibility.
Makes theme selection more intuitive.
Provides a more polished user experience.

### 👀 Have you spent some time to check if this issue has been raised before?

- [x] I checked and didn't find similar issue

### 🏢 Have you read the Code of Conduct?

- [x] I have read the [Code of Conduct](https://github.com/appwrite/.github/blob/main/CODE_OF_CONDUCT.md)

Start by locating the existing theme selector in the Console UI and reviewing how its current selection state, keyboard navigation, and screen-reader behavior work. Done means Light, Dark, and System options have icons, clearer selected-state feedback, keyboard and screen-reader support, and descriptive tooltips.

Written by the indexing model from the issue text.