Adopt shared WebUI layout primitives

webui/src/features/postbox/PostboxPage.tsx

@@ -18,7 +18,7 @@ import {
   UserRoundCheck,
   X
 } from "lucide-react";
-import {
+import { FormGrid,
   ActionBlockerHint,
   Button,
   ConfirmDialog,
@@ -938,7 +938,7 @@ export default function PostboxPage({
           </div>
         }
       >
-        <div className="postbox-form-grid">
+        <FormGrid gap="small" className="postbox-form-grid">
           <FormField label="Name" documentation={POSTBOX_FIELD_DOCUMENTATION}>
             <input
               value={groupingDraft.name}
@@ -962,7 +962,7 @@ export default function PostboxPage({
               }
             />
           </div>
-        </div>
+        </FormGrid>
         <fieldset className="postbox-source-selector">
           <legend>Source postboxes</legend>
           {postboxes.map((postbox) => (
@@ -1014,7 +1014,7 @@ export default function PostboxPage({
           </div>
         }
       >
-        <div className="postbox-compose-grid">
+        <FormGrid columns={2} gap="small" collapseAt="narrow" className="postbox-compose-grid">
           <FormField label="Postbox" documentation={POSTBOX_FIELD_DOCUMENTATION}>
             <select
               value={messageDraft.postbox_id}
@@ -1099,7 +1099,7 @@ export default function PostboxPage({
               />
             </FormField>
           </div>
-        </div>
+        </FormGrid>
       </Dialog>
 
       <ConfirmDialog