feat: 优化情景切换功能

frontend/src/App.css

@@ -1674,6 +1674,54 @@ body {
   line-height: 1.4;
 }
 
+/* ---- Scenario Hint Card (非空状态顶部提示) ---- */
+
+.chat-panel__scenario-hint {
+  display: flex;
+  justify-content: center;
+  padding: 12px 24px 0;
+}
+
+.scenario-hint-card {
+  display: flex;
+  align-items: center;
+  gap: 12px;
+  padding: 12px 16px;
+  border-radius: var(--radius-sm);
+  background: linear-gradient(135deg, rgba(59, 130, 246, 0.08) 0%, rgba(99, 102, 241, 0.08) 100%);
+  border: 1px solid rgba(59, 130, 246, 0.2);
+  max-width: 520px;
+  width: 100%;
+  box-shadow: 0 2px 8px rgba(59, 130, 246, 0.06);
+}
+
+.scenario-hint-card__icon {
+  font-size: 1.8rem;
+  flex-shrink: 0;
+  filter: drop-shadow(0 2px 4px rgba(0, 0, 0, 0.1));
+}
+
+.scenario-hint-card__text {
+  display: flex;
+  flex-direction: column;
+  gap: 4px;
+  flex: 1;
+}
+
+.scenario-hint-card__text strong {
+  font-size: 0.9rem;
+  font-weight: 600;
+  color: var(--color-text);
+  letter-spacing: 0.01em;
+}
+
+.scenario-hint-card__text p {
+  font-size: 0.75rem;
+  color: var(--color-text-muted);
+  line-height: 1.5;
+  margin: 0;
+}
+
 /* ---- Voice Input Button ---- */
 
 .chat-input__voice {