fix(network): consolidate import buttons, redesign empty state, add shortcut overlay

- Import/Export button in editor header: removed standalone Import button, moved
  draw.io import into Export/Import dropdown with labelled sections; fixes
  conceptual trap where Import implied operating on the current diagram
- List page: replaced two identical Upload-icon Import buttons with a single
  dropdown (Import JSON / Import draw.io) with format descriptions
- Empty state: replaced icon-in-box with a horizontal card featuring a static
  SVG topology preview, MSP-specific value prop, and dual CTAs
- Keyboard shortcuts: new KeyboardShortcutsOverlay component (4-group grid),
  triggered by ? key or the ? button pinned to the canvas bottom-right corner;
  wired into useCanvasShortcuts hook
- Fixed Share2 → FileOutput icon for draw.io export (Share2 = send to someone,
  FileOutput = export file format)

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

frontend/src/components/network/hooks/useCanvasShortcuts.ts

@@ -37,6 +37,7 @@ export function useCanvasShortcuts({
   onRedo,
   onNudge,
   onSetMode,
+  onToggleShortcuts,
 }: {
   nodes: Node[]
   edges: Edge[]
@@ -48,6 +49,7 @@ export function useCanvasShortcuts({
   onRedo: () => void
   onNudge: (dx: number, dy: number) => void
   onSetMode: (mode: 'select' | 'pan' | 'connect') => void
+  onToggleShortcuts: () => void
 }) {
   const { getNodes, fitView, screenToFlowPosition, setNodes: rfSetNodes } = useReactFlow()
   const clipboardRef = useRef<ClipboardData | null>(null)
@@ -279,12 +281,15 @@ export function useCanvasShortcuts({
       } else if (e.key === '[' && !ctrl) {
         e.preventDefault()
         sendSelectedToBack()
+      } else if (e.key === '?' && !ctrl) {
+        e.preventDefault()
+        onToggleShortcuts()
       }
     }
 
     document.addEventListener('keydown', handleKeyDown)
     return () => document.removeEventListener('keydown', handleKeyDown)
-  }, [copyNodes, pasteNodes, duplicateNodes, selectAll, fitView, bringSelectedToFront, sendSelectedToBack, onUndo, onRedo, onNudge, onSetMode])
+  }, [copyNodes, pasteNodes, duplicateNodes, selectAll, fitView, bringSelectedToFront, sendSelectedToBack, onUndo, onRedo, onNudge, onSetMode, onToggleShortcuts])
 
   return {
     copyNodes,