feat: Enhance AgentChatWidget with Chat Session Management and UI Improvements

- Implemented functionality to save, load, and delete chat sessions, allowing users to manage their chat history effectively.
- Introduced a new layout for the connection status bar and improved the AI provider selection interface for better visibility.
- Enhanced the UI of the AgentEditor with a modular 3-column layout, including dedicated tabs for system prompts and common tiles, improving user experience and organization.

src/app/editor/agent/agent_chat_widget.h

@@ -244,17 +244,25 @@ public:
   struct ChatSession {
     std::string id;
     std::string name;
+    std::filesystem::path save_path;
     cli::agent::ConversationalAgentService agent_service;
     size_t last_history_size = 0;
     bool history_loaded = false;
     bool history_dirty = false;
     std::filesystem::path history_path;
+    absl::Time created_at = absl::Now();
     absl::Time last_persist_time = absl::InfinitePast();
     
     ChatSession(const std::string& session_id, const std::string& session_name)
         : id(session_id), name(session_name) {}
   };
   
+  void SaveChatSession(const ChatSession& session);
+  void LoadChatSession(const std::string& session_id);
+  void DeleteChatSession(const std::string& session_id);
+  std::vector<std::string> GetSavedSessions();
+  std::filesystem::path GetSessionsDirectory();
+  
   std::vector<ChatSession> chat_sessions_;
   int active_session_index_ = 0;