Theme switcher dropdown in UI

[tacheraSasi]

Feature: Theme Switcher Dropdown

Add a dropdown in the floating actions to switch themes without restarting the server.

Details

• Replace the simple dark/light toggle with a dropdown menu
• List all available themes from the themes package
• Apply theme changes client-side instantly via CSS class swapping
• Persist theme selection in localStorage
• Optionally fetch re-rendered content from the server with the new theme
• Show theme preview thumbnails or color swatches in the dropdown

Why

Users want to try different themes without restarting. A live theme switcher makes the tool feel more polished.