Implement synchronization, offline data handling, and intelligent request routing

- Add services for offline data management, including `offline-data.service.ts`, ensuring data is saved to a local database and synced with the server when online.
- Introduce bidirectional `SyncService` for managing data synchronization with conflict resolution and retry mechanisms.
- Create `data.service.ts` to handle smart routing between local database and server API based on connectivity status.
- Update models and logic to support comprehensive synchronization for books, chapters, characters, and conversations.
- Implement event listeners for online/offline detection and automatic sync scheduling.

lib/db/sync.service.ts

@@ -0,0 +1,58 @@
+/**
+ * Sync progress interface
+ */
+export interface SyncProgress {
+    isSyncing: boolean;
+    pendingChanges: number;
+    isOnline: boolean;
+    lastError?: string;
+}
+
+/**
+ * Get sync status from local database
+ */
+export async function getSyncStatus(): Promise<SyncProgress> {
+    if (!window.electron) {
+        return {
+            isSyncing: false,
+            pendingChanges: 0,
+            isOnline: navigator.onLine
+        };
+    }
+
+    try {
+        const result = await window.electron.dbGetSyncStatus();
+        if (!result.success) {
+            throw new Error(result.error);
+        }
+        return result.data;
+    } catch (error) {
+        console.error('Failed to get sync status:', error);
+        return {
+            isSyncing: false,
+            pendingChanges: 0,
+            isOnline: navigator.onLine,
+            lastError: error instanceof Error ? error.message : 'Unknown error'
+        };
+    }
+}
+
+/**
+ * Get pending changes to sync
+ */
+export async function getPendingChanges(limit: number = 100) {
+    if (!window.electron) {
+        return [];
+    }
+
+    try {
+        const result = await window.electron.dbGetPendingChanges(limit);
+        if (!result.success) {
+            throw new Error(result.error);
+        }
+        return result.data || [];
+    } catch (error) {
+        console.error('Failed to get pending changes:', error);
+        return [];
+    }
+}