feat(walkthrough): add Getting Started tour with first-run invite

Bespoke spotlight walkthrough (no new deps) that guides new users through the
main features. A version-stamped invite bubble offers the tour to new users and
re-offers it once to existing users after an upgrade; restartable from Settings.

- Multi-hole SVG-mask overlay keeps a ringed control and any open modal lit
- Steps: scan, scan history, inventory, nodes, edit, text/zone, grouping,
  style, imports, and a closing recap with a GitHub link
- Scan/history/inventory/import steps inject demo data (no backend calls) and
  are filtered out in standalone, which ends on a full-mode recap
- Persisted invite state via localStorage (utils/walkthrough.ts), live tour
  state via zustand (stores/walkthroughStore.ts)

ha-relevant: maybe

frontend/src/utils/__tests__/walkthrough.test.ts

@@ -0,0 +1,66 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest'
+import {
+  WALKTHROUGH_VERSION,
+  readWalkthrough,
+  writeWalkthrough,
+  subscribeWalkthrough,
+  shouldOfferWalkthrough,
+  markWalkthroughSeen,
+  resetWalkthrough,
+} from '../walkthrough'
+
+beforeEach(() => localStorage.clear())
+
+describe('readWalkthrough', () => {
+  it('defaults to pending / unseen when nothing is stored', () => {
+    expect(readWalkthrough()).toEqual({ seenVersion: null, status: 'pending' })
+  })
+
+  it('round-trips written state', () => {
+    writeWalkthrough({ seenVersion: 3, status: 'completed' })
+    expect(readWalkthrough()).toEqual({ seenVersion: 3, status: 'completed' })
+  })
+
+  it('falls back to defaults on malformed json', () => {
+    localStorage.setItem('homelable.walkthrough', '{not json')
+    expect(readWalkthrough()).toEqual({ seenVersion: null, status: 'pending' })
+  })
+})
+
+describe('shouldOfferWalkthrough', () => {
+  it('offers a brand-new user (seenVersion null)', () => {
+    expect(shouldOfferWalkthrough()).toBe(true)
+  })
+
+  it('offers an upgrader whose seen version is behind', () => {
+    writeWalkthrough({ seenVersion: WALKTHROUGH_VERSION - 1, status: 'completed' })
+    expect(shouldOfferWalkthrough()).toBe(true)
+  })
+
+  it('stops offering once the current version is stamped', () => {
+    markWalkthroughSeen('completed')
+    expect(shouldOfferWalkthrough()).toBe(false)
+    expect(readWalkthrough().seenVersion).toBe(WALKTHROUGH_VERSION)
+  })
+})
+
+describe('resetWalkthrough', () => {
+  it('re-offers after a reset', () => {
+    markWalkthroughSeen('skipped')
+    expect(shouldOfferWalkthrough()).toBe(false)
+    resetWalkthrough()
+    expect(shouldOfferWalkthrough()).toBe(true)
+  })
+})
+
+describe('subscribeWalkthrough', () => {
+  it('notifies listeners on write and unsubscribes cleanly', () => {
+    const spy = vi.fn()
+    const unsub = subscribeWalkthrough(spy)
+    writeWalkthrough({ seenVersion: 1, status: 'completed' })
+    expect(spy).toHaveBeenCalledWith({ seenVersion: 1, status: 'completed' })
+    unsub()
+    writeWalkthrough({ seenVersion: 2, status: 'skipped' })
+    expect(spy).toHaveBeenCalledTimes(1)
+  })
+})