feat(dashboard): add morning brief preview and test-send action

src/gateway/ui/style.css

@@ -1571,6 +1571,47 @@ tr:hover td {
   color: var(--text-primary);
 }
 
+.assistant-preview {
+  margin-top: 12px;
+  padding: 12px;
+  border: 1px solid var(--border);
+  border-radius: var(--radius);
+  background-color: var(--bg-secondary);
+}
+
+.assistant-preview-header {
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+  gap: 8px;
+  margin-bottom: 10px;
+}
+
+.assistant-preview-title {
+  font-size: var(--font-size-sm);
+  font-weight: 700;
+  color: var(--text-primary);
+}
+
+.assistant-preview-meta {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 10px;
+  margin-bottom: 10px;
+}
+
+.assistant-preview-body {
+  max-height: 180px;
+  overflow-y: auto;
+  border: 1px solid var(--border-light);
+  border-radius: var(--radius);
+  padding: 10px;
+  background-color: var(--bg-tertiary);
+  white-space: pre-wrap;
+  word-break: break-word;
+  font-size: var(--font-size-sm);
+}
+
 /* ── Responsive: Mobile ─────────────────────────────────────── */
 
 @media (max-width: 768px) {