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/login/+page.svelte

@@ -10,9 +10,6 @@ const activeLocale = $derived(getLocale().toUpperCase());
 </script>
 
 <div class="relative flex min-h-screen flex-col bg-canvas">
-	<!-- DGB purple accent strip -->
-	<div class="h-1 bg-brand-purple"></div>
-
 	<!-- Language switcher -->
 	<div class="absolute top-4 right-4 flex items-center gap-1">
 		{#each locales as locale (locale)}