fix(#96): remove off-brand lavender accent bar from all pages

The h-1 bg-brand-purple strip (#b4b9ff) is not a De Gruyter brand
color and was added as a rough placeholder. Removed from +layout.svelte
and the three auth pages (login, forgot-password, reset-password).
Also removed the unused --palette-purple and --color-brand-purple CSS
tokens from layout.css.

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

frontend/src/routes/forgot-password/+page.svelte

@@ -5,9 +5,6 @@ let { form }: { form?: { error?: string; success?: boolean } } = $props();
 </script>
 
 <div class="relative flex min-h-screen flex-col bg-surface">
-	<!-- Accent strip -->
-	<div class="h-1 bg-brand-purple"></div>
-
 	<div class="flex flex-1 items-center justify-center px-4">
 		<div class="w-full max-w-sm">
 			<!-- Logo -->