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)

app/onboarding/page.tsx

@@ -47,7 +47,7 @@ export default function OnboardingPage() {
     <div className="min-h-screen flex items-center justify-center bg-neutral-50 dark:bg-neutral-900 px-4">
       <div className="max-w-md w-full space-y-8">
         <div className="text-center">
-          <h2 className="mt-6 text-2xl font-bold text-neutral-900 dark:text-white">
+          <h2 className="mt-6 text-2xl font-bold text-white">
             Welcome to Pulse
           </h2>
           <p className="mt-2 text-sm text-neutral-600 dark:text-neutral-400">