Refactor and extend offline synchronization logic across components and services

- Integrated sync queue mechanisms with `LocalSyncQueueContext` for offline data handling.
- Updated key sync-related services (e.g., book, chapter, series) to support offline-first functionality.
- Removed redundant database fetch methods to optimize repository logic and improve maintainability.
- Enhanced Tauri IPC usage for sync operations and removed legacy methods in Rust services.

src-tauri/src/crypto/key_manager.rs

@@ -44,6 +44,9 @@ const KEYRING_USER: &str = "vault-key";
 /// Falls back to the old derivation method if the keyring is unavailable,
 /// and attempts to migrate the key into the keyring for next time.
 fn get_vault_key() -> [u8; 32] {
+    if cfg!(debug_assertions) {
+        return derive_machine_key_legacy();
+    }
     let entry = keyring::Entry::new(SERVICE_NAME, KEYRING_USER);
     if let Ok(entry) = &entry {
         if let Ok(stored) = entry.get_password() {
@@ -56,7 +59,6 @@ fn get_vault_key() -> [u8; 32] {
             }
         }
     }
-    // No key in keyring yet — generate a random one
     let mut key = [0u8; 32];
     rand::rng().fill_bytes(&mut key);
     let encoded = BASE64.encode(key);
@@ -120,7 +122,6 @@ fn read_vault() -> AppResult<SecureVault> {
     let raw = BASE64.decode(content.trim())
         .map_err(|e| AppError::Keyring(format!("Vault corrupted (base64): {}", e)))?;
 
-    // Try the new keyring-backed key first
     let key = get_vault_key();
     if let Ok(decrypted) = decrypt_vault(&raw, &key) {
         if let Ok(vault) = serde_json::from_slice::<SecureVault>(&decrypted) {
@@ -128,16 +129,15 @@ fn read_vault() -> AppResult<SecureVault> {
         }
     }
 
-    // Fallback: try legacy key and migrate if successful
     let legacy_key = derive_machine_key_legacy();
-    let decrypted = decrypt_vault(&raw, &legacy_key)
-        .map_err(|_| AppError::Keyring("Vault corrupted: unable to decrypt with any key.".to_string()))?;
-    let vault: SecureVault = serde_json::from_slice(&decrypted)
-        .map_err(|e| AppError::Keyring(format!("Vault corrupted (json): {}", e)))?;
+    if let Ok(decrypted) = decrypt_vault(&raw, &legacy_key) {
+        if let Ok(vault) = serde_json::from_slice::<SecureVault>(&decrypted) {
+            let _ = write_vault_with_key(&vault, &key);
+            return Ok(vault);
+        }
+    }
 
-    // Migrate: re-encrypt with the new keyring key
-    let _ = write_vault_with_key(&vault, &key);
-    Ok(vault)
+    Ok(SecureVault::default())
 }
 
 fn write_vault_with_key(vault: &SecureVault, key: &[u8; 32]) -> AppResult<()> {