feat: Dashboard 接真 API，题图渐变占位，排播占位，近9期收视柱图，颜色按年度目标判定

frontend/src/pages/Dashboard/Dashboard.css

@@ -34,6 +34,33 @@
   justify-content: center;
   color: var(--color-primary-green);
   font-size: 12px;
+  position: relative;
+  overflow: hidden;
+}
+
+.banner-placeholder-gradient {
+  position: absolute;
+  inset: 0;
+  background: linear-gradient(135deg, #6b8e6b 0%, #a8c89a 50%, #d4e6b5 100%);
+}
+
+.change-cover-btn {
+  position: absolute;
+  bottom: 6px;
+  right: 6px;
+  z-index: 2;
+}
+
+.placeholder-label {
+  position: absolute;
+  top: 50%;
+  left: 50%;
+  transform: translate(-50%, -50%);
+  z-index: 2;
+  color: rgba(255,255,255,0.85);
+  font-size: 13px;
+  font-weight: 600;
+  letter-spacing: 2px;
 }
 
 /* KPI Cards */