feat: create a new canvas by copying an existing one

Add a 'Copy from existing' option to the New Canvas modal. It lists every
canvas with node/group/text counts; picking one deep-copies its nodes, edges,
parent/child links and canvas state (viewport, custom style, floor plan) into
a fresh design.

Backend: POST /designs/{source_id}/copy remaps node ids, re-points edges and
parent links, and clones canvas state; GET /designs now returns per-design
counts for the picker. Standalone mode clones the localStorage canvas.

Closes #216

ha-relevant: maybe

frontend/src/api/client.ts

@@ -169,6 +169,8 @@ export const designsApi = {
   list: () => api.get<import('@/types').Design[]>('/designs'),
   create: (data: { name: string; icon?: string; design_type?: string }) =>
     api.post<import('@/types').Design>('/designs', data),
+  copy: (sourceId: string, data: { name: string; icon?: string }) =>
+    api.post<import('@/types').Design>(`/designs/${sourceId}/copy`, data),
   update: (id: string, data: { name?: string; icon?: string }) =>
     api.put<import('@/types').Design>(`/designs/${id}`, data),
   delete: (id: string) => api.delete(`/designs/${id}`),