Refactor code structure for improved readability and maintainability

Co-authored-by: Copilot <copilot@github.com>

lib/db.ts

@@ -24,18 +24,27 @@ export interface Task {
   created_at: string;
 }
 
+export interface ChatMessage {
+  id: string;
+  role: "user" | "assistant";
+  content: string;
+  created_at: string;
+}
+
 export class MindSpaceDB extends Dexie {
   journals!: Table<Journal>;
   tasks!: Table<Task>;
+  chat_messages!: Table<ChatMessage>;
 
   constructor() {
     super("MindSpaceDatabase");
 
     // Schema tanımlamaları.
     // IndexedDB'de sadece indekslenecek (üzerinde arama/sıralama yapılacak) alanları belirtiriz.
-    this.version(1).stores({
+    this.version(2).stores({
       journals: "id, date, mood",
       tasks: "id, status, date, journal_id",
+      chat_messages: "id, created_at",
     });
   }
 }