Adopt shared WebUI structural primitives

webui/src/features/postbox/PostboxAdminPanel.tsx

@@ -26,6 +26,7 @@ import { FormGrid,
   SegmentedControl,
   SelectionList,
   SelectionListItem,
+  StatePanel,
   StatusBadge,
   ToggleSwitch,
   i18nMessage,
@@ -800,11 +801,7 @@ function TemplateWorkspace({
             </div>
           </>
         ) : (
-          <div className="postbox-empty">
-            <Boxes size={24} />
-            <strong>Select a template</strong>
-            <p>Published revisions lazily resolve stable unit-specific addresses.</p>
-          </div>
+          <StatePanel size="fill" icon={<Boxes size={24} />} title="Select a template" description="Published revisions lazily resolve stable unit-specific addresses." />
         )}
       </section>
     </div>
@@ -878,11 +875,7 @@ function PostboxWorkspace({
             </dl>
           </>
         ) : (
-          <div className="postbox-empty">
-            <Inbox size={24} />
-            <strong>Select a Postbox</strong>
-            <p>Materialized Postboxes remain durable through vacancy and reassignment.</p>
-          </div>
+          <StatePanel size="fill" icon={<Inbox size={24} />} title="Select a Postbox" description="Materialized Postboxes remain durable through vacancy and reassignment." />
         )}
       </section>
     </div>