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/components/theme-provider.tsx

@@ -71,3 +71,13 @@ export function useSwitchToDarkThemeOnMount() {
     setTheme(ThemeEnum.Dark);
   }, [setTheme]);
 }
+
+export function useSyncThemeFromParams(theme: string | null) {
+  const { setTheme } = useTheme();
+
+  useEffect(() => {
+    if (theme && (theme === ThemeEnum.Light || theme === ThemeEnum.Dark)) {
+      setTheme(theme as ThemeEnum);
+    }
+  }, [theme, setTheme]);
+}