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/pages/ChatsPage.tsx

@@ -42,7 +42,7 @@ export function ChatsPage() {
                 Back
               </button>
               <div className="min-w-0">
-                <p className="truncate text-sm font-semibold">{activeChat?.title || `@${me?.username}`}</p>
+                <p className="truncate text-sm font-semibold">{activeChat?.display_title || activeChat?.title || me?.name || `@${me?.username}`}</p>
                 <p className="truncate text-xs text-slate-300/80">{activeChat ? activeChat.type : "Select a chat"}</p>
               </div>
             </div>