Adopt shared WebUI layout primitives

webui/src/features/postbox/PostboxAdminPanel.tsx

@@ -1,3 +1,4 @@
+import { MetricGrid } from "@govoplan/core-webui";
 import { useCallback, useEffect, useMemo, useState } from "react";
 import {
   Archive,
@@ -1370,13 +1371,13 @@ function TemplateImpactPreview({
         </div>
         {preview.truncated ? <StatusBadge status="warning" label="List truncated" /> : null}
       </div>
-      <div className="metric-grid compact postbox-preview-metrics">
+      <MetricGrid columns="auto" density="compact" spacing="none" minimum="compact">
         <MetricCard label="Targets" value={preview.total} tone="info" />
         <MetricCard label="Ready" value={preview.ready_count} tone="good" />
         <MetricCard label="Existing" value={preview.existing_count} />
         <MetricCard label="Vacant" value={preview.vacant_count} tone="warning" />
         <MetricCard label="Blocked" value={preview.blocked_count} tone="danger" />
-      </div>
+      </MetricGrid>
       {preview.diagnostics.length ? (
         <p className="postbox-form-note">{preview.diagnostics.join(", ")}</p>
       ) : null}