feat: support multi-canvas (designs) in frontend-only standalone mode

Standalone mode (VITE_STANDALONE=true) skipped the designs system entirely:
the design list stayed empty (switcher hidden) and all canvases collapsed onto
a single localStorage key. Add a localStorage-backed design layer mirroring the
backend designs API.

- standaloneStorage util: list/create/update/delete designs + per-design canvas
  storage (homelable_designs + homelable_canvas:<id>). ensureSeed migrates a
  legacy single-canvas install into a default design so existing data survives.
- App.tsx: load/seed designs, save + switch canvases per design id in standalone.
- Sidebar.tsx: create/update/delete dispatch to standaloneStorage when standalone.
- Tests for the new storage util.

ha-relevant: no

frontend/src/utils/standaloneStorage.ts

@@ -0,0 +1,112 @@
+/**
+ * Standalone-mode persistence (VITE_STANDALONE=true).
+ *
+ * No backend is available, so designs (multi-canvas) and their canvas data are
+ * persisted directly to localStorage:
+ *   - `homelable_designs`         → Design[] (the canvas list)
+ *   - `homelable_canvas:<id>`     → { nodes, edges, theme_id, custom_style } per design
+ *
+ * Legacy single-canvas installs stored everything under `homelable_canvas`
+ * (no per-design key, no design list). `ensureSeed()` migrates that data into a
+ * default design on first run so existing users keep their canvas.
+ */
+import type { Node, Edge } from '@xyflow/react'
+import type { Design, DesignType, NodeData, EdgeData, CustomStyleDef } from '@/types'
+import { generateUUID } from '@/utils/uuid'
+
+const DESIGNS_KEY = 'homelable_designs'
+const LEGACY_CANVAS_KEY = 'homelable_canvas'
+const canvasKey = (designId: string) => `${LEGACY_CANVAS_KEY}:${designId}`
+
+export interface StandaloneCanvas {
+  nodes: Node<NodeData>[]
+  edges: Edge<EdgeData>[]
+  theme_id?: string
+  custom_style?: CustomStyleDef | null
+}
+
+function readJSON<T>(key: string): T | null {
+  try {
+    const raw = localStorage.getItem(key)
+    return raw ? (JSON.parse(raw) as T) : null
+  } catch {
+    return null
+  }
+}
+
+function nowIso(): string {
+  return new Date().toISOString()
+}
+
+/** Read the design list. Returns [] when none have been created yet. */
+export function listDesigns(): Design[] {
+  return readJSON<Design[]>(DESIGNS_KEY) ?? []
+}
+
+function writeDesigns(designs: Design[]): void {
+  localStorage.setItem(DESIGNS_KEY, JSON.stringify(designs))
+}
+
+/**
+ * Guarantee at least one design exists and return the full list.
+ * Migrates a legacy single-canvas install into a default design on first run.
+ */
+export function ensureSeed(): Design[] {
+  const existing = listDesigns()
+  if (existing.length > 0) return existing
+
+  const design: Design = {
+    id: generateUUID(),
+    name: 'My Homelab',
+    design_type: 'network',
+    icon: null,
+    created_at: nowIso(),
+    updated_at: nowIso(),
+  }
+  writeDesigns([design])
+
+  // Migrate legacy canvas data (stored under the bare key) into this design.
+  const legacy = readJSON<StandaloneCanvas>(LEGACY_CANVAS_KEY)
+  if (legacy && localStorage.getItem(canvasKey(design.id)) === null) {
+    localStorage.setItem(canvasKey(design.id), JSON.stringify(legacy))
+    localStorage.removeItem(LEGACY_CANVAS_KEY)
+  }
+  return [design]
+}
+
+export function createDesign(name: string, icon?: string | null, design_type: DesignType = 'network'): Design {
+  const design: Design = {
+    id: generateUUID(),
+    name,
+    design_type,
+    icon: icon ?? null,
+    created_at: nowIso(),
+    updated_at: nowIso(),
+  }
+  writeDesigns([...listDesigns(), design])
+  return design
+}
+
+export function updateDesign(id: string, patch: Partial<Pick<Design, 'name' | 'icon'>>): Design | null {
+  const designs = listDesigns()
+  const idx = designs.findIndex((d) => d.id === id)
+  if (idx === -1) return null
+  const updated: Design = { ...designs[idx], ...patch, updated_at: nowIso() }
+  designs[idx] = updated
+  writeDesigns(designs)
+  return updated
+}
+
+export function deleteDesign(id: string): void {
+  writeDesigns(listDesigns().filter((d) => d.id !== id))
+  localStorage.removeItem(canvasKey(id))
+}
+
+/** Load a design's canvas. Returns null when the design has never been saved. */
+export function loadCanvas(designId: string): StandaloneCanvas | null {
+  return readJSON<StandaloneCanvas>(canvasKey(designId))
+}
+
+export function saveCanvas(designId: string, data: StandaloneCanvas): void {
+  localStorage.setItem(canvasKey(designId), JSON.stringify(data))
+}