chats: add chat avatars and profile view-only modal

docs/api-reference.md

@@ -182,6 +182,7 @@ All fields are optional.
   "public_id": "A1B2C3D4E5F6G7H8J9K0L1M2",
   "type": "private",
   "title": null,
+  "avatar_url": null,
   "display_title": "Other User",
   "handle": null,
   "description": null,
@@ -198,6 +199,7 @@ All fields are optional.
   "counterpart_user_id": 2,
   "counterpart_name": "Other User",
   "counterpart_username": "other",
+  "counterpart_avatar_url": "https://...",
   "counterpart_is_online": true,
   "counterpart_last_seen_at": "2026-03-08T10:00:00Z",
   "last_message_text": "Hello",
@@ -662,6 +664,26 @@ Body:
 
 Response: `200` + `ChatRead`
 
+### PATCH `/api/v1/chats/{chat_id}/profile`
+
+Auth required.  
+Body (all fields optional):
+
+```json
+{
+  "title": "New title",
+  "description": "optional description",
+  "avatar_url": "https://..."
+}
+```
+
+Rules:
+
+- only for `group`/`channel`
+- only for `owner`/`admin`
+
+Response: `200` + `ChatRead`
+
 ### GET `/api/v1/chats/{chat_id}/members`
 
 Auth required.