feat: add user display profiles and fix web context menu UX

backend:

- add required user name and optional bio fields

- extend auth/register and user schemas/services with name/bio

- add alembic migration 0006 with safe backfill name=username

- compute per-user chat display_title for private chats

- keep Saved Messages delete-for-all protections

web:

- registration now includes name

- add profile edit modal (name/username/bio/avatar url)

- show private chat names via display_title

- fix context menus to open near cursor with viewport clamping

- stabilize +/close floating button to remove visual jump

web/src/api/users.ts

@@ -1,5 +1,5 @@
 import { http } from "./http";
-import type { UserSearchItem } from "../chat/types";
+import type { AuthUser, UserSearchItem } from "../chat/types";
 
 export async function searchUsers(query: string, limit = 20): Promise<UserSearchItem[]> {
   const { data } = await http.get<UserSearchItem[]>("/users/search", {
@@ -7,3 +7,15 @@ export async function searchUsers(query: string, limit = 20): Promise<UserSearch
   });
   return data;
 }
+
+interface UserProfileUpdatePayload {
+  name?: string;
+  username?: string;
+  bio?: string | null;
+  avatar_url?: string | null;
+}
+
+export async function updateMyProfile(payload: UserProfileUpdatePayload): Promise<AuthUser> {
+  const { data } = await http.put<AuthUser>("/users/profile", payload);
+  return data;
+}