fix: frontend consistency audit — 55 files cleaned up

Consistency fixes:
- Extract getThisWeekRange/getThisMonthRange to shared lib/utils/dateRanges.ts
  (removed 4 identical copy-pasted definitions)
- Add error boundaries for behavior, cdn, search, pagespeed pages
  (4 new error.tsx files — previously fell through to generic parent error)
- Add "View setup guide" CTA to empty states on journeys and behavior pages
  (previously showed text with no actionable button)
- Fix non-lazy useState initializer in funnel detail page
- Fix Bot & Spam settings header from text-xl to text-2xl (matches all other sections)
- Add useMinimumLoading to PageSpeed skeleton (consistent with all other pages)

Cleanup:
- Remove 438 redundant dark: class prefixes (app is dark-mode only)
  text-neutral-500 dark:text-neutral-400 → text-neutral-400 (206 occurrences)
  text-neutral-900 dark:text-white → text-white (232 occurrences)
- Remove dead @stripe/react-stripe-js and @stripe/stripe-js packages
  (billing migrated to Polar, no code imports Stripe)
- Remove duplicate motion package (framer-motion is the one actually used)

components/settings/SettingsModalWrapper.tsx

@@ -72,7 +72,7 @@ function NotificationCenterPlaceholder() {
   return (
     <div className="text-center max-w-md mx-auto py-8">
       <BellIcon className="w-12 h-12 text-neutral-300 mx-auto mb-4" />
-      <h3 className="text-lg font-medium text-neutral-900 dark:text-white mb-2">Notification Center</h3>
+      <h3 className="text-lg font-medium text-white mb-2">Notification Center</h3>
       <p className="text-sm text-neutral-500 mb-4">View and manage all your notifications in one place.</p>
       <Link href="/notifications" className="inline-flex items-center gap-2 px-4 py-2 bg-brand-orange text-white rounded-lg hover:bg-brand-orange/90 transition-colors">
         Open Notification Center