Stop the GL account-code check from being able to abort startup

The catch filter only matched DomainException/OperationCanceledException,
so anything else escaped a hosted service and took the host down. One such
path was mine: `"items": null` deserializes over the property initialiser,
so `data.Items.Select(...)` would NullReferenceException straight past the
filter.

Fixed at both ends - the reader tolerates a null or absent items array, and
the catch is now a deliberate catch-all that only lets a genuine host
shutdown through. This is a diagnostic; a diagnostic that can core-dump the
thing it diagnoses is worse than no diagnostic at all.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Backend/ERPCore/Services/GeneralLedgerService.cs

@@ -56,9 +56,10 @@ public sealed class GeneralLedgerService : IGeneralLedgerService
         // `parentAccountId` is a field, not a nested `children` array).
         var response = await _client.SendAsync(HttpMethod.Get, "accounts", null, null, null, ct);
         var data = ParseSuccess<GlAccountListData>(response);
-        return data.Items
+        // `"items": null` deserializes over the property initialiser, so the null check is load-bearing.
+        return (data.Items ?? [])
+            .Where(a => a is not null && !string.IsNullOrWhiteSpace(a.AccountCode))
             .Select(a => a.AccountCode)
-            .Where(c => !string.IsNullOrWhiteSpace(c))
             .ToHashSet(StringComparer.Ordinal);
     }
 
@@ -111,7 +112,7 @@ public sealed class GeneralLedgerService : IGeneralLedgerService
 
     private sealed class GlAccountListData
     {
-        public List<GlAccountData> Items { get; set; } = [];
+        public List<GlAccountData>? Items { get; set; }
     }
 
     private sealed class GlAccountData