feat(contacts): switch contacts UX to email-first flow

- expose email in user search/contact responses

- add add-contact-by-email API endpoint

- show email in contacts/search and add contact form by email in Contacts tab

web/src/chat/types.ts

@@ -100,6 +100,7 @@ export interface UserSearchItem {
   id: number;
   name: string;
   username: string;
+  email?: string;
   avatar_url: string | null;
 }