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/activity-feed.tsx

@@ -40,7 +40,7 @@ function getActionLabel(action: string, t: (key: string) => string | undefined):
 function timeAgo(dateStr: string, t: (key: string) => string | undefined): string {
   const diff = Date.now() - new Date(dateStr).getTime()
   const mins = Math.floor(diff / 60000)
-  if (mins < 1) return t('brainstorm.justNow') || 'just now'
+  if (mins < 1) return t('brainstorm.justNow')
   if (mins < 60) return `${mins}m`
   const hours = Math.floor(mins / 60)
   if (hours < 24) return `${hours}h`
@@ -63,7 +63,7 @@ export function ActivityFeed({ activities, isOpen, onToggle, t }: ActivityFeedPr
             <div className="flex items-center gap-2">
               <Activity size={14} className="text-orange-500" />
               <h3 className="text-[10px] font-bold uppercase tracking-[0.2em] text-foreground">
-                {t('brainstorm.activityTitle') || 'Activity'}
+                {t('brainstorm.activityTitle')}
               </h3>
             </div>
             <button
@@ -78,7 +78,7 @@ export function ActivityFeed({ activities, isOpen, onToggle, t }: ActivityFeedPr
             {activities.length === 0 ? (
               <div className="p-6 text-center">
                 <p className="text-xs italic text-muted-foreground">
-                  {t('brainstorm.noActivity') || 'No activity yet'}
+                  {t('brainstorm.noActivity')}
                 </p>
               </div>
             ) : (

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" />