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/utils/__tests__/alignmentSettings.test.ts

@@ -0,0 +1,42 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest'
+import {
+  DEFAULT_ALIGNMENT_SETTINGS,
+  readAlignmentSettings,
+  writeAlignmentSettings,
+  subscribeAlignmentSettings,
+} from '../alignmentSettings'
+
+describe('alignmentSettings', () => {
+  beforeEach(() => {
+    localStorage.clear()
+  })
+
+  it('returns defaults when nothing stored', () => {
+    expect(readAlignmentSettings()).toEqual(DEFAULT_ALIGNMENT_SETTINGS)
+  })
+
+  it('roundtrips through localStorage', () => {
+    writeAlignmentSettings({ enabled: false, threshold: 10 })
+    expect(readAlignmentSettings()).toEqual({ enabled: false, threshold: 10 })
+  })
+
+  it('falls back to defaults when stored value is corrupted', () => {
+    localStorage.setItem('homelable.alignmentGuides', '{not json')
+    expect(readAlignmentSettings()).toEqual(DEFAULT_ALIGNMENT_SETTINGS)
+  })
+
+  it('fills missing fields from defaults', () => {
+    localStorage.setItem('homelable.alignmentGuides', JSON.stringify({ enabled: false }))
+    expect(readAlignmentSettings()).toEqual({ enabled: false, threshold: DEFAULT_ALIGNMENT_SETTINGS.threshold })
+  })
+
+  it('notifies subscribers on write', () => {
+    const listener = vi.fn()
+    const unsubscribe = subscribeAlignmentSettings(listener)
+    writeAlignmentSettings({ enabled: false, threshold: 8 })
+    expect(listener).toHaveBeenCalledWith({ enabled: false, threshold: 8 })
+    unsubscribe()
+    writeAlignmentSettings({ enabled: true, threshold: 4 })
+    expect(listener).toHaveBeenCalledTimes(1)
+  })
+})