feat(canvas): alignment guides + snap while dragging nodes

Draw.io / Figma style: while dragging a node, show dashed cyan guide
lines when its edges (left / center / right / top / middle / bottom)
align with another node's, and snap the position to the matched line
within a configurable threshold.

- utils/alignment.ts: pure snap math, returns delta + guide segments.
  Same-size boxes show all aligned guides simultaneously.
- canvas/AlignmentGuides.tsx: SVG overlay locked to the React Flow
  viewport (panned/zoomed correctly).
- hooks/useAlignmentGuides.ts: wires onNodeDrag/onNodeDragStop, applies
  snap via setNodes, listens for Alt to temporarily disable.
- utils/alignmentSettings.ts: localStorage-backed prefs (enabled,
  threshold 2-16px) with a tiny CustomEvent pub-sub so the SettingsPanel
  and the drag hook stay in sync without a global store.
- Sidebar settings panel: toggle + threshold slider.

Multi-selection drag uses the union bounding box. Children with
parentId are skipped for v1 to avoid mixing absolute and parent-relative
coordinates. Hold Alt to escape snap.

frontend/src/components/canvas/AlignmentGuides.tsx

@@ -0,0 +1,70 @@
+import { useViewport } from '@xyflow/react'
+import type { Guide } from '@/utils/alignment'
+
+interface AlignmentGuidesProps {
+  guides: Guide[]
+  color?: string
+}
+
+/**
+ * SVG overlay that draws alignment guide lines on top of the React Flow canvas.
+ * Coordinates are in canvas (flow) space; we read the viewport transform to
+ * project them into screen space so lines stay locked to nodes when the user
+ * pans or zooms.
+ */
+export function AlignmentGuides({ guides, color = '#00d4ff' }: AlignmentGuidesProps) {
+  const { x: vx, y: vy, zoom } = useViewport()
+
+  if (guides.length === 0) return null
+
+  return (
+    <svg
+      style={{
+        position: 'absolute',
+        inset: 0,
+        width: '100%',
+        height: '100%',
+        pointerEvents: 'none',
+        zIndex: 5,
+        overflow: 'visible',
+      }}
+    >
+      {guides.map((g, i) => {
+        if (g.axis === 'x') {
+          const x = g.position * zoom + vx
+          const y1 = g.start * zoom + vy
+          const y2 = g.end * zoom + vy
+          return (
+            <line
+              key={`x-${i}-${g.position}`}
+              x1={x}
+              y1={y1}
+              x2={x}
+              y2={y2}
+              stroke={color}
+              strokeWidth={1}
+              strokeDasharray="4 3"
+              shapeRendering="crispEdges"
+            />
+          )
+        }
+        const y = g.position * zoom + vy
+        const x1 = g.start * zoom + vx
+        const x2 = g.end * zoom + vx
+        return (
+          <line
+            key={`y-${i}-${g.position}`}
+            x1={x1}
+            y1={y}
+            x2={x2}
+            y2={y}
+            stroke={color}
+            strokeWidth={1}
+            strokeDasharray="4 3"
+            shapeRendering="crispEdges"
+          />
+        )
+      })}
+    </svg>
+  )
+}

frontend/src/components/canvas/CanvasContainer.tsx

@@ -20,6 +20,8 @@ import { THEMES } from '@/utils/themes'
 import { nodeTypes } from './nodes/nodeTypes'
 import { edgeTypes } from './edges/edgeTypes'
 import { SearchBar } from './SearchBar'
+import { AlignmentGuides } from './AlignmentGuides'
+import { useAlignmentGuides } from '@/hooks/useAlignmentGuides'
 import type { NodeData, EdgeData } from '@/types'
 
 interface CanvasContainerProps {
@@ -83,6 +85,8 @@ export function CanvasContainer({ onConnect: onConnectProp, onEdgeDoubleClick, o
     []
   )
 
+  const { guides, onNodeDrag, onNodeDragStop } = useAlignmentGuides()
+
   return (
     <div className="w-full h-full" style={{ background: theme.colors.canvasBackground }}>
       <ReactFlow
@@ -96,6 +100,8 @@ export function CanvasContainer({ onConnect: onConnectProp, onEdgeDoubleClick, o
         onEdgeDoubleClick={handleEdgeDoubleClick}
         onNodeDoubleClick={handleNodeDoubleClick}
         onNodeDragStart={onNodeDragStart}
+        onNodeDrag={onNodeDrag}
+        onNodeDragStop={onNodeDragStop}
         nodeTypes={nodeTypes}
         edgeTypes={edgeTypes}
         deleteKeyCode={['Backspace', 'Delete']}
@@ -121,6 +127,7 @@ export function CanvasContainer({ onConnect: onConnectProp, onEdgeDoubleClick, o
           color={theme.colors.canvasDotColor}
         />
         <SearchBar onOpenPending={onOpenPending} />
+        <AlignmentGuides guides={guides} />
         <Controls>
           <ControlButton
             onClick={() => setLassoMode((m) => !m)}

frontend/src/components/canvas/__tests__/AlignmentGuides.test.tsx

@@ -0,0 +1,47 @@
+import { describe, it, expect, vi } from 'vitest'
+import { render } from '@testing-library/react'
+import { AlignmentGuides } from '../AlignmentGuides'
+import type { Guide } from '@/utils/alignment'
+
+vi.mock('@xyflow/react', () => ({
+  useViewport: () => ({ x: 50, y: 100, zoom: 2 }),
+}))
+
+describe('AlignmentGuides', () => {
+  it('renders nothing when no guides', () => {
+    const { container } = render(<AlignmentGuides guides={[]} />)
+    expect(container.querySelector('svg')).toBeNull()
+  })
+
+  it('projects an x-axis guide through the viewport transform', () => {
+    const guides: Guide[] = [{ axis: 'x', position: 100, start: 0, end: 200 }]
+    const { container } = render(<AlignmentGuides guides={guides} />)
+    const line = container.querySelector('line')!
+    // x = position * zoom + vx → 100*2 + 50 = 250
+    expect(line.getAttribute('x1')).toBe('250')
+    expect(line.getAttribute('x2')).toBe('250')
+    // y1 = start * zoom + vy → 0*2 + 100 = 100; y2 = 200*2 + 100 = 500
+    expect(line.getAttribute('y1')).toBe('100')
+    expect(line.getAttribute('y2')).toBe('500')
+  })
+
+  it('projects a y-axis guide horizontally', () => {
+    const guides: Guide[] = [{ axis: 'y', position: 50, start: 10, end: 60 }]
+    const { container } = render(<AlignmentGuides guides={guides} />)
+    const line = container.querySelector('line')!
+    // y = 50*2 + 100 = 200; x1 = 10*2 + 50 = 70; x2 = 60*2 + 50 = 170
+    expect(line.getAttribute('y1')).toBe('200')
+    expect(line.getAttribute('y2')).toBe('200')
+    expect(line.getAttribute('x1')).toBe('70')
+    expect(line.getAttribute('x2')).toBe('170')
+  })
+
+  it('renders one line per guide', () => {
+    const guides: Guide[] = [
+      { axis: 'x', position: 100, start: 0, end: 200 },
+      { axis: 'y', position: 50, start: 10, end: 60 },
+    ]
+    const { container } = render(<AlignmentGuides guides={guides} />)
+    expect(container.querySelectorAll('line')).toHaveLength(2)
+  })
+})