Improve error handling and safety when retrieving account seed words

- Update `Account.seedWords()` to use `runCatching` and return an empty string instead of a nullable value on failure.
- Simplify seed word handling in `AccountBackupDialog` and `AccountExportService` by removing null checks.

app/src/main/java/com/greenart7c3/nostrsigner/models/Account.kt

@@ -120,7 +120,7 @@ class Account(
         return signer.signerSync.decrypt(encryptedContent, fromPublicKey)
     }
 
-    suspend fun seedWords() = DataStoreAccess.getEncryptedKey(Amber.instance, npub, DataStoreAccess.SEED_WORDS)
+    suspend fun seedWords() = runCatching { DataStoreAccess.getEncryptedKey(Amber.instance, npub, DataStoreAccess.SEED_WORDS) }.getOrNull() ?: ""
 
     suspend fun decryptZapEvent(
         data: String,

app/src/main/java/com/greenart7c3/nostrsigner/service/AccountExportService.kt

@@ -121,7 +121,7 @@ object AccountExportService {
      */
     private suspend fun accountToExportData(account: Account, password: String): AccountExportData {
         val seedWords = account.seedWords()
-        val encrypedSeedWords = if (seedWords != null && seedWords.isNotBlank()) account.nip44Encrypt(seedWords, account.hexKey) else ""
+        val encrypedSeedWords = if (seedWords.isNotBlank()) account.nip44Encrypt(seedWords, account.hexKey) else ""
         val encryptedNsec = account.nip49Encrypt(password)
 
         return AccountExportData(

app/src/main/java/com/greenart7c3/nostrsigner/ui/actions/AccountBackupDialog.kt

@@ -227,7 +227,7 @@ fun AccountBackupScreen(
                             launch(Dispatchers.IO) {
                                 localAccount = LocalPreferences.loadFromEncryptedStorage(Amber.instance, it.npub)
                                 localAccount?.let { acc ->
-                                    seedWords = acc.seedWords() ?: ""
+                                    seedWords = acc.seedWords()
                                 }
                             }
                         }