Sessions: edit form, soft-delete, lazy creation, chat as separate page

- Session click opens edit form (name, description, image, agent) instead of chat
- Chat is now a separate route (#chat/{id}), opened via button
- Sessions created on save, not on click
- Soft-delete for sessions (is_deleted flag)
- Delete button in session form
- Removed title double-click editing from chat
- Card images now square (aspect-ratio: 1)
- New Session as placeholder card

Models/SessionRecord.cs

@@ -5,7 +5,10 @@ public class SessionRecord
     public long Id { get; set; }
     public string SessionId { get; set; }
     public string Name { get; set; }
+    public string Description { get; set; }
+    public string Image { get; set; }
     public string AgentId { get; set; }
+    public bool IsDeleted { get; set; }
     public string CreatedAt { get; set; }
     public string UpdatedAt { get; set; }
 }