docs: add comprehensive Stripe billing guide

Covers architecture, configuration steps, user flows, API routes,
webhooks, pricing, testing with Stripe CLI, production checklist,
and troubleshooting.

memento-note/components/brainstorm/brainstorm-page.tsx

@@ -289,7 +289,7 @@ export function BrainstormPage() {
             </motion.div>
             <div className="flex-1">
               <h1 className="text-4xl font-serif font-medium text-foreground tracking-tight">
-                {t('brainstorm.title') || 'Waves of Thought'}
+                {t('brainstorm.title')}
               </h1>
               <div className="flex items-center gap-2 mt-1">
                 <span className="w-8 h-px bg-brand-accent/40" />