fix: disable context picker mode after selecting a scope

When a context scope is picked via the pipette, the picker mode now
turns off automatically since the selection is done and shows as a chip.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/app/composables/usePilotChat.ts

@@ -271,6 +271,7 @@ export function usePilotChat(opts: {
       return;
     }
     contextScopes.value = [...contextScopes.value, scope];
+    contextPickerEnabled.value = false;
   }
 
   function removeContextScope(scope: ContextScope) {