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/types/index.ts

@@ -8,6 +8,10 @@ export interface Design {
   icon?: string | null
   created_at: string
   updated_at: string
+  /** Populated by the design list endpoint for the "copy from existing" picker. */
+  node_count?: number | null
+  group_count?: number | null
+  text_count?: number | null
 }
 
 export type NodeType =