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/dashboard/SiteNav.tsx

@@ -46,7 +46,7 @@ export default function SiteNav({ siteId }: SiteNavProps) {
             tabIndex={isActive(tab.href) ? 0 : -1}
             className={`relative shrink-0 whitespace-nowrap px-3 py-2 text-sm font-medium transition-colors focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-brand-orange rounded-t cursor-pointer -mb-px ${
               isActive(tab.href)
-                ? 'text-neutral-900 dark:text-white'
+                ? 'text-white'
                 : 'text-neutral-400 dark:text-neutral-500 hover:text-neutral-700 dark:hover:text-neutral-300'
             }`}
           >