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/walkthrough.ts

@@ -0,0 +1,73 @@
+/**
+ * Getting Started walkthrough — persisted invite state (localStorage).
+ *
+ * Mirrors the autosaveSettings.ts pattern: a `homelable.<name>` key plus a
+ * `window` CustomEvent for same-tab sync, so App and SettingsModal stay aligned
+ * without a global store.
+ *
+ * `seenVersion` is stamped with WALKTHROUGH_VERSION when the user finishes or
+ * dismisses the tour. Bump WALKTHROUGH_VERSION whenever new steps ship so
+ * existing users are re-offered the tour exactly once (they may have missed the
+ * new features). A brand-new user starts at seenVersion=null and is offered too.
+ */
+export const WALKTHROUGH_VERSION = 1
+
+export type WalkthroughStatus = 'pending' | 'skipped' | 'completed'
+
+export interface WalkthroughState {
+  seenVersion: number | null
+  status: WalkthroughStatus
+}
+
+const KEY = 'homelable.walkthrough'
+const EVENT = 'homelable:walkthrough-changed'
+
+export const DEFAULT_WALKTHROUGH_STATE: WalkthroughState = { seenVersion: null, status: 'pending' }
+
+function isStatus(v: unknown): v is WalkthroughStatus {
+  return v === 'pending' || v === 'skipped' || v === 'completed'
+}
+
+export function readWalkthrough(): WalkthroughState {
+  try {
+    const raw = localStorage.getItem(KEY)
+    if (!raw) return { ...DEFAULT_WALKTHROUGH_STATE }
+    const parsed = JSON.parse(raw) as Partial<WalkthroughState>
+    return {
+      seenVersion: typeof parsed.seenVersion === 'number' ? parsed.seenVersion : null,
+      status: isStatus(parsed.status) ? parsed.status : 'pending',
+    }
+  } catch {
+    return { ...DEFAULT_WALKTHROUGH_STATE }
+  }
+}
+
+export function writeWalkthrough(state: WalkthroughState): void {
+  try {
+    localStorage.setItem(KEY, JSON.stringify(state))
+    window.dispatchEvent(new CustomEvent(EVENT, { detail: state }))
+  } catch {
+    // ignore quota / SSR
+  }
+}
+
+export function subscribeWalkthrough(listener: (s: WalkthroughState) => void): () => void {
+  const handler = (e: Event) => listener((e as CustomEvent<WalkthroughState>).detail)
+  window.addEventListener(EVENT, handler)
+  return () => window.removeEventListener(EVENT, handler)
+}
+
+/** True when the invite should be offered (new user, or upgraded past the last seen version). */
+export function shouldOfferWalkthrough(state: WalkthroughState = readWalkthrough()): boolean {
+  return state.seenVersion !== WALKTHROUGH_VERSION
+}
+
+/** Stamp the current version so the invite stops auto-showing until the next bump. */
+export function markWalkthroughSeen(status: Exclude<WalkthroughStatus, 'pending'>): void {
+  writeWalkthrough({ seenVersion: WALKTHROUGH_VERSION, status })
+}
+
+/** Clear the stamp so the tour is offered again (used by "Restart" in Settings). */
+export function resetWalkthrough(): void {
+  writeWalkthrough({ ...DEFAULT_WALKTHROUGH_STATE })
+}