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.css

@@ -10,7 +10,6 @@
 	--palette-mint: #a1dcd8;
 	--palette-turquoise: #00c7b1;
 	--palette-sand: #f0efe9;
-	--palette-purple: #b4b9ff;
 
 	/* Typography */
 	--font-sans: 'Montserrat', ui-sans-serif, system-ui, sans-serif;
@@ -58,7 +57,6 @@
 	--color-pdf-text: var(--c-pdf-text);
 
 	/* Static brand tokens (not themed) */
-	--color-brand-purple: var(--palette-purple);
 	--color-brand-navy: var(--palette-navy);
 	--color-brand-mint: var(--palette-mint);
 }