Remove PostHog analytics and update dependencies to latest minor versions

- Remove posthog-js and posthog-node dependencies
- Delete PostHogProvider component and lib/posthog.ts
- Remove PostHog configuration from environment files
- Remove PostHog rewrites from next.config.mjs
- Update all npm subdependencies to latest minor versions
- Update @types/react and @types/react-dom overrides to 19.2.9 and 19.2.3

app/components/WhatsNewPopup.tsx

@@ -47,7 +47,6 @@ const CHANGELOG = [
       'Enhanced Home page layout and content',
       'Themes updated to use OKLCH (from HSL)',
       'All themes updated (light themes look similar)',
-      'Caching system added (incomplete)',
       'Skeleton loading added across all pages'
     ],
     fixes: [