feat: mark embedded session chat as desktop surface

Adds a surface=desktop URL param to embedded session chat links
Uses the surface override to classify embedded chat as desktop
Covers the new URL parameter in tests

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

@@ -54,6 +54,7 @@ describe('embedded session chat URL', () => {
 
     const url = new URL(src);
     expect(url.searchParams.get('ocPanel')).toBe('session-chat');
+    expect(url.searchParams.get('surface')).toBe('desktop');
     expect(url.searchParams.get('themeMode')).toBe('system');
     expect(url.searchParams.get('themeVariant')).toBe('dark');
     expect(url.searchParams.get('lightThemeId')).toBe('custom-light');

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

@@ -30,6 +30,7 @@ export const buildEmbeddedSessionChatURL = (
 
   const url = new URL(window.location.pathname, window.location.origin);
   url.searchParams.set('ocPanel', 'session-chat');
+  url.searchParams.set('surface', 'desktop');
   url.searchParams.set('sessionId', sessionID);
   if (readOnly) {
     url.searchParams.set('readOnly', '1');