fix(exchange-rate-guard): evitar SSR de CreateEditDialog con {#if browser}

Dialog.Portal intenta instanciar clases del DOM durante SSR causando
'Class constructors cannot be invoked without new' en GET /dashboard.
El diálogo es puramente client-side, no necesita renderizado en servidor.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/components/dashboard/exchange_rate/exchange-rate-guard.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
 	import { onMount } from 'svelte';
+	import { browser } from '$app/environment';
 	import { companyStore } from '$lib/stores/company.svelte';
 	import { uiStore } from '$lib/stores/ui.svelte';
 	import { getExchangeRates } from '$lib/api/dashboard/a76/general_catalogs/exchange-rate';
@@ -69,4 +70,6 @@
 	});
 </script>
 
-<CreateEditDialog bind:open onSuccess={handleSuccess} {overlayClass} />
+{#if browser}
+	<CreateEditDialog bind:open onSuccess={handleSuccess} {overlayClass} />
+{/if}