fix: Remove false 'Template-API not available' message on 404

When no template was saved yet, the API returns 404. This does not mean
the API is unavailable - it just means no template exists yet. The code
was incorrectly setting isTemplateApiAvailable to false on 404, causing
the misleading message 'Template-API auf diesem Server nicht verfuegbar'.

Fixed in both InvoiceTemplatePage and ReportTemplatePage.

frontend/src/pages/InvoiceTemplatePage.tsx

@@ -1225,7 +1225,6 @@ export default function InvoiceTemplatePage() {
             setResizingElementId(null);
             setTemplateUpdatedAt(null);
             setTemplateError(null);
-            setIsTemplateApiAvailable(false);
             return;
           }
           setTemplateError((error as Error).message);