feat: canvas history (undo/redo), copy/paste nodes, node search, shortcuts modal

- Undo/Redo (Ctrl+Z / Ctrl+Y): 50-entry snapshot stack in canvasStore; snapshot before all mutations and on node drag stop
- Copy/Paste (Ctrl+C / Ctrl+V): copy selected nodes to clipboard, paste with +50px offset and new IDs
- Node search (Ctrl+K): spotlight overlay — fuzzy search by label/IP/hostname, jumps + focuses matched node
- Shortcuts modal (?): lists all keyboard shortcuts, accessible via ? key or toolbar ? button
- Toolbar: undo/redo buttons (disabled when stack empty), ? help button

frontend/src/components/panels/Toolbar.tsx

@@ -1,4 +1,4 @@
-import { Save, LayoutDashboard, Download, Palette } from 'lucide-react'
+import { Save, LayoutDashboard, Download, Palette, Undo2, Redo2, HelpCircle } from 'lucide-react'
 import { Button } from '@/components/ui/button'
 import { Logo } from '@/components/ui/Logo'
 import { useCanvasStore } from '@/stores/canvasStore'
@@ -8,15 +8,37 @@ interface ToolbarProps {
   onAutoLayout: () => void
   onExport: () => void
   onChangeStyle: () => void
+  onUndo: () => void
+  onRedo: () => void
+  onShortcuts: () => void
 }
 
-export function Toolbar({ onSave, onAutoLayout, onExport, onChangeStyle }: ToolbarProps) {
-  const { hasUnsavedChanges } = useCanvasStore()
+export function Toolbar({ onSave, onAutoLayout, onExport, onChangeStyle, onUndo, onRedo, onShortcuts }: ToolbarProps) {
+  const { hasUnsavedChanges, past, future } = useCanvasStore()
 
   return (
     <header className="flex items-center gap-2 px-4 py-2 border-b border-border bg-[#161b22] shrink-0">
       <Logo size={28} showText={true} />
       <div className="flex-1" />
+      <Button
+        size="sm" variant="ghost"
+        className="gap-1.5 text-muted-foreground hover:text-foreground disabled:opacity-30"
+        onClick={onUndo}
+        disabled={past.length === 0}
+        title="Undo (Ctrl+Z)"
+      >
+        <Undo2 size={14} />
+      </Button>
+      <Button
+        size="sm" variant="ghost"
+        className="gap-1.5 text-muted-foreground hover:text-foreground disabled:opacity-30"
+        onClick={onRedo}
+        disabled={future.length === 0}
+        title="Redo (Ctrl+Y)"
+      >
+        <Redo2 size={14} />
+      </Button>
+      <div className="w-px h-4 bg-border mx-1" />
       <Button size="sm" variant="ghost" className="gap-1.5 text-muted-foreground hover:text-foreground" onClick={onAutoLayout}>
         <LayoutDashboard size={14} /> Auto Layout
       </Button>
@@ -26,6 +48,9 @@ export function Toolbar({ onSave, onAutoLayout, onExport, onChangeStyle }: Toolb
       <Button size="sm" variant="ghost" className="gap-1.5 text-muted-foreground hover:text-foreground" onClick={onExport}>
         <Download size={14} /> Export
       </Button>
+      <Button size="sm" variant="ghost" className="gap-1.5 text-muted-foreground hover:text-foreground" onClick={onShortcuts} title="Keyboard shortcuts (?)">
+        <HelpCircle size={14} />
+      </Button>
       <Button
         size="sm"
         className="gap-1.5 relative"