fix: remove main-panel padding gap + add box-sizing for scroll cutoff

Two layout issues:
1. Gap above content: .main-panel had padding: 0.25em which created
   a visible gap above views. Removed — views control their own
   internal padding.

2. Scroll cutoff at bottom: .main-panel > * had height: 100% but
   no box-sizing: border-box. Views with their own padding (like
   config-page) overflowed because padding added to the 100% height.
   Added box-sizing: border-box to the global rule so padding is
   included in the height calculation.

frontend/index.css

@@ -142,7 +142,6 @@ body div.sidebar {
 .main-panel {
     flex: 1;
     min-width: 0;
-    padding: 0.25em;
     background-color: var(--yj-bg-surface, #212529);
     overflow: hidden;
     contain: layout style paint;
@@ -150,6 +149,7 @@ body div.sidebar {
 
 .main-panel > * {
     height: 100%;
+    box-sizing: border-box;
     contain: layout style paint;
 }