feat(mazenet): persist canvas layout per topology to localStorage

Dragging a LAN or decky, or resizing a NetBox, updates React state
but previously vanished on reload because the grid-layout adapter
rewrote everything from the graph. Add a per-topology localStorage
snapshot (key: mazenet.layout.<topologyId>) that captures net
x/y/w/h and decky x/y; useLayoutPersistor writes it debounced, and
getTopology merges it over adaptTopology's grid so entities without
a stored entry still fall back to a clean auto-layout. Deleting a
topology calls clearLayout to drop its snapshot.

decnet_web/src/components/TopologyList/TopologyList.tsx

@@ -2,6 +2,7 @@ import React, { useCallback, useEffect, useState } from 'react';
 import { useNavigate } from 'react-router-dom';
 import { Network, Plus, Power, Trash2, UploadCloud, RefreshCw } from 'lucide-react';
 import api from '../../utils/api';
+import { clearLayout } from '../MazeNET/useMazeLayoutStore';
 import './TopologyList.css';
 
 interface TopologySummary {
@@ -90,6 +91,7 @@ const TopologyList: React.FC = () => {
     setBusy(id);
     try {
       await api.delete(`/topologies/${id}`);
+      clearLayout(id);
       await fetchRows();
     } catch (e) {
       setErr((e as Error)?.message ?? 'delete failed');