fix(ui): bound dashboard height so panels don't overflow viewport

.content-viewport is overflow-y: auto so flex:1 on dash-grid grew to
content height. Fix: dashboard uses height:100% instead of min-height,
and :has(>.dashboard) disables content-viewport scroll only on that
route — all other pages keep their normal scroll.

decnet_web/src/components/Dashboard.css

@@ -2,7 +2,7 @@
   display: flex;
   flex-direction: column;
   gap: 24px;
-  min-height: 100%;
+  height: 100%;
 }
 
 /* Page header */

decnet_web/src/components/Layout.css

@@ -346,3 +346,8 @@
   padding: 32px;
   overflow-y: auto;
 }
+
+/* Dashboard fills the viewport without scrolling — panels scroll internally */
+.content-viewport:has(> .dashboard) {
+  overflow: hidden;
+}