Fix checkbox/label spacing on Notifications and other form checkboxes

Tabler's form-check-single modifier zeroes margin on .form-check-input,
which cancels the -2rem margin-inline-start the base .form-check rule
relies on to offset the checkbox into its label's padding gutter. That
left checkboxes floating almost flush against their label text.

Removed the form-check-single class from all 9 affected checkboxes:
6 in NotificationSettings, 1 each in DnsProviderForm, IntegrationForm,
and IntegrationEditForm.

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

web/src/components/IntegrationForm.tsx

@@ -119,7 +119,7 @@ export default function IntegrationForm({ onCreated, onCancel }: { onCreated: ()
           {fields.map((field) => (
             <div className="col-md-4" key={field.key}>
               {field.type === "checkbox" ? (
-                <label className="form-check form-check-single mt-4">
+                <label className="form-check mt-4">
                   <input
                     type="checkbox"
                     className="form-check-input"