[AI] Fix SimpleFIN batch sync error_code TypeError

Fix "Cannot read properties of undefined (reading 'error_code')" that
occurs during SimpleFIN batch sync by:

1. Adding null check for downloadSimpleFinTransactions result in
   simpleFinBatchSync (sync.ts) - the function can return undefined
   when user token is missing

2. Adding .catch() handler on individual processBankSyncDownload
   promises so a single account failure doesn't crash the entire
   batch via Promise.all rejection

3. Using optional chaining on syncResponse.res?.error_code in app.ts
   and handling the case where res is undefined with proper error
   reporting

https://claude.ai/code/session_01XbHgxxrXYR3UTyW6VmYj47

packages/loot-core/src/server/accounts/app.ts

@@ -1069,7 +1069,7 @@ async function simpleFinBatchSync({
       const matchedTransactions: Array<TransactionEntity['id']> = [];
       const updatedAccounts: Array<AccountEntity['id']> = [];
 
-      if (syncResponse.res.error_code) {
+      if (syncResponse.res?.error_code) {
         errors.push(
           handleSyncError(
             {
@@ -1081,7 +1081,7 @@ async function simpleFinBatchSync({
             account,
           ),
         );
-      } else {
+      } else if (syncResponse.res) {
         const syncResponseData = await handleSyncResponse(
           syncResponse.res,
           account,
@@ -1090,6 +1090,15 @@ async function simpleFinBatchSync({
         newTransactions.push(...syncResponseData.newTransactions);
         matchedTransactions.push(...syncResponseData.matchedTransactions);
         updatedAccounts.push(...syncResponseData.updatedAccounts);
+      } else {
+        errors.push(
+          handleSyncError(
+            new Error(
+              'Failed syncing account "' + account.name + '": empty response',
+            ),
+            account,
+          ),
+        );
       }
 
       retVal.push({

packages/loot-core/src/server/accounts/sync.ts

@@ -1092,6 +1092,16 @@ export async function simpleFinBatchSync(
     startDates,
   );
 
+  if (!res) {
+    return accounts.map(account => ({
+      accountId: account.id,
+      res: {
+        error_type: 'NO_DATA',
+        error_code: 'NO_DATA',
+      },
+    }));
+  }
+
   const promises = [];
   for (let i = 0; i < accounts.length; i++) {
     const account = accounts[i];
@@ -1126,12 +1136,18 @@ export async function simpleFinBatchSync(
     }
 
     promises.push(
-      processBankSyncDownload(download, account.id, acctRow, newAccount).then(
-        res => ({
+      processBankSyncDownload(download, account.id, acctRow, newAccount)
+        .then(res => ({
           accountId: account.id,
           res,
-        }),
-      ),
+        }))
+        .catch(err => ({
+          accountId: account.id,
+          res: {
+            error_type: err?.category || 'INTERNAL_ERROR',
+            error_code: err?.code || 'INTERNAL_ERROR',
+          },
+        })),
     );
   }