polish(network): visual refinements across node, edge, and panel components

- DeviceNode: flat bg-card (no surface gradient), darker icon plate inset,
  correct text-muted token for category label
- GroupNode: label pill gets bg-card/90 background so it reads against canvas
- ConnectionEdge: label now has border + bg-card so it doesn't float invisible
- BaseHandle: tightened to 12px with accent-toned border
- NodeStatusIndicator: glow reduced to 0.15 opacity (design system compliant)
- ContextMenu: Ungroup now uses Ungroup icon instead of BoxSelect
- DeviceToolbar: group type icons coloured with semantic palette
- PropertiesPanel: empty state gets icon tile + cleaner copy hierarchy
- DiagramEditor: shortcut ? button repositioned above MiniMap, accent hover
- NetworkDiagrams list: card thumbnail placeholder uses dot-grid pattern,
  card menu gets icons and divider before destructive action

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/network/edges/ConnectionEdge.tsx

@@ -54,7 +54,7 @@ function ConnectionEdgeComponent(props: EdgeProps) {
       {props.label && (
         <EdgeLabelRenderer>
           <div
-            className="nodrag nopan rounded bg-page px-1.5 py-0.5 text-[10px] text-muted-foreground"
+            className="nodrag nopan rounded border border-default bg-card px-1.5 py-0.5 text-[10px] text-muted-foreground shadow-sm"
             style={{
               position: 'absolute',
               transform: `translate(-50%, -50%) translate(${labelX}px, ${labelY}px)`,