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/ui/node-status-indicator.tsx

@@ -11,9 +11,9 @@ const STATUS_BORDER_COLORS: Record<NodeStatus, string> = {
 }
 
 const STATUS_GLOW: Record<NodeStatus, string> = {
-  online: 'shadow-[0_0_8px_rgba(52,211,153,0.3)]',
-  offline: 'shadow-[0_0_8px_rgba(248,113,113,0.3)]',
-  degraded: 'shadow-[0_0_8px_rgba(250,204,21,0.3)]',
+  online: 'shadow-[0_0_6px_rgba(52,211,153,0.15)]',
+  offline: 'shadow-[0_0_6px_rgba(248,113,113,0.15)]',
+  degraded: 'shadow-[0_0_6px_rgba(250,204,21,0.15)]',
   unknown: '',
 }