fix(login): use bg-canvas instead of bg-surface for page background

The login page used bg-surface (white) as its outer background.
The global layout already has bg-canvas (sand), so using bg-surface
created a visible white layer with a mismatched color.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/login/+page.svelte

@@ -9,7 +9,7 @@ const localeMap = { DE: 'de', EN: 'en', ES: 'es' } as const;
 const activeLocale = $derived(getLocale().toUpperCase());
 </script>
 
-<div class="relative flex min-h-screen flex-col bg-surface">
+<div class="relative flex min-h-screen flex-col bg-canvas">
 	<!-- DGB purple accent strip -->
 	<div class="h-1 bg-brand-purple"></div>