fix(ui): compact embedded chat URLs (#2431)

* fix(ui): keep embedded session chat URLs compact

* test(ui): guard embedded session chat URL length

* fix(ui): sync embedded custom themes

Bootstrap custom themes from parent window so iframe URLs stay compact.

Refs #2423

* docs(ui): clarify theme sync precedence

packages/ui/src/components/layout/contextPanelEmbeddedChat.ts

@@ -46,7 +46,6 @@ export const buildEmbeddedSessionChatURL = (
   url.searchParams.set('lightThemeId', theme.lightThemeId);
   url.searchParams.set('darkThemeId', theme.darkThemeId);
   url.searchParams.set('themeVariant', theme.currentTheme.metadata.variant === 'dark' ? 'dark' : 'light');
-  url.searchParams.set('currentTheme', JSON.stringify(theme.currentTheme));
 
   url.hash = '';
   return url.toString();
@@ -126,4 +125,4 @@ export const getEmbeddedSessionChatOriginSessionId = (): string | null => {
   } catch {
     return null;
   }
-};
+};