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

app/chats/schemas.py

@@ -11,6 +11,7 @@ class ChatRead(BaseModel):
     id: int
     type: ChatType
     title: str | None = None
+    display_title: str | None = None
     handle: str | None = None
     description: str | None = None
     is_public: bool = False