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/nodes/GroupNode.tsx

@@ -51,7 +51,7 @@ const GroupNodeComponent = ({ data, selected, id }: NodeProps) => {
           boxSizing: 'border-box',
         }}
       >
-        <div className="absolute top-0 left-0 -translate-y-full pb-0.5 pl-1">
+        <div className="absolute top-0 left-2 -translate-y-full pb-0.5">
           {editing ? (
             <input
               ref={inputRef}
@@ -62,12 +62,12 @@ const GroupNodeComponent = ({ data, selected, id }: NodeProps) => {
                 if (e.key === 'Enter' || e.key === 'Escape') handleLabelCommit()
                 e.stopPropagation()
               }}
-              className="text-[11px] font-medium bg-transparent border-none outline-none text-primary min-w-[40px] max-w-[200px]"
+              className="rounded-sm px-1.5 py-0.5 text-[11px] font-semibold bg-card/90 border-none outline-none min-w-[40px] max-w-[200px]"
               style={{ color }}
             />
           ) : (
             <span
-              className="text-[11px] font-medium cursor-text select-none"
+              className="inline-block rounded-sm bg-card/90 px-1.5 py-0.5 text-[11px] font-semibold cursor-text select-none tracking-wide"
               style={{ color }}
               onDoubleClick={() => setEditing(true)}
             >