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/chat/types.ts

@@ -6,6 +6,7 @@ export interface Chat {
   id: number;
   type: ChatType;
   title: string | null;
+  display_title?: string | null;
   handle?: string | null;
   description?: string | null;
   is_public?: boolean;
@@ -36,7 +37,9 @@ export interface Message {
 export interface AuthUser {
   id: number;
   email: string;
+  name: string;
   username: string;
+  bio?: string | null;
   avatar_url: string | null;
   email_verified: boolean;
   created_at: string;
@@ -51,6 +54,7 @@ export interface TokenPair {
 
 export interface UserSearchItem {
   id: number;
+  name: string;
   username: string;
   avatar_url: string | null;
 }