fix(merge-plan): snapshot proposal prop to avoid structuredClone proxy throw

structuredClone on a Svelte 5 reactive Proxy throws DataCloneError during
component init, causing MergeProposalPanel to silently fail to mount.
Replace with \$state.snapshot which is the documented way to deep-copy a
reactive prop into a local editable state.

web/src/lib/components/admin/MergeProposalPanel.svelte

@@ -1,5 +1,4 @@
 <script lang="ts">
-	import { untrack } from 'svelte';
 	import Button from '$lib/components/ui/Button.svelte';
 	import type { AdminMarketDetail, DuplicateMarket, MarketMergeProposal } from '$lib/api/types.js';
 	import { fieldLabels, formatValue } from './fieldRenderers.js';
@@ -16,7 +15,7 @@
 	let { proposal, candidate, current, applying, onApply, onClose }: Props = $props();
 
 	// Local copy the admin can edit before applying.
-	let edited = $state(untrack(() => structuredClone(proposal)));
+	let edited = $state($state.snapshot(proposal));
 
 	const fieldOrder = Object.keys(edited.field_merges ?? {});