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/+layout.svelte

@@ -39,9 +39,6 @@ const userInitials = $derived.by(() => {
 <div class="min-h-screen bg-canvas" data-hydrated={hydrated || undefined}>
 	{#if !isAuthPage}
 		<header class="sticky top-0 z-50 border-b border-line-2 bg-surface">
-			<!-- De Gruyter Brill purple accent strip -->
-			<div class="h-1 bg-brand-purple"></div>
-
 			<div class="mx-auto max-w-7xl px-4 sm:px-6 lg:px-8">
 				<div class="flex h-16 justify-between">
 					<!-- Logo & Nav -->