Feature: embedded chat theme (#11581)

### What problem does this PR solve?

This PR closing feature request #11286. 
It implements ability to choose the background theme of the _Full screen
chat_ which is Embed into webpage.
Looks like that:
<img width="501" height="349" alt="image"
src="https://github.com/user-attachments/assets/e5fdfb14-9ed9-43bb-a40d-4b580985b9d4"
/>

It works similar to `Locale`, using url parameter to set the theme.
if the parameter is invalid then is using the default theme.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

---------

Co-authored-by: Your Name <you@example.com>

web/src/pages/next-chats/hooks/use-send-shared-message.ts

@@ -33,6 +33,7 @@ export const useGetSharedChatSearchParams = () => {
     from: searchParams.get('from') as SharedFrom,
     sharedId: searchParams.get('shared_id'),
     locale: searchParams.get('locale'),
+    theme: searchParams.get('theme'),
     data: data,
     visibleAvatar: searchParams.get('visible_avatar')
       ? searchParams.get('visible_avatar') !== '1'