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/auth.ts

@@ -1,8 +1,8 @@
 import { http } from "./http";
 import type { AuthUser, TokenPair } from "../chat/types";
 
-export async function registerRequest(email: string, username: string, password: string): Promise<void> {
-  await http.post("/auth/register", { email, username, password });
+export async function registerRequest(email: string, name: string, username: string, password: string): Promise<void> {
+  await http.post("/auth/register", { email, name, username, password });
 }
 
 export async function loginRequest(email: string, password: string): Promise<TokenPair> {