Blob examples should use access: 'private' instead of 'public'

[vvo]

Summary

The blob examples currently default to access: 'public', but we're moving toward private-by-default for blob stores. These examples are what developers copy-paste from, so they should reflect the recommended default.

Files to update

1. storage/blob-starter/components/uploader.tsx (line 32)

   - access: 'public',
   + access: 'private',

2. storage/blob-sveltekit/src/routes/+page.server.ts (line 14)

   - access: 'public',
   + access: 'private',

Context

• CLI PRs already in progress: feat(cli): require --access flag for blob commands (breaking) vercel#15375, feat(cli): blob command improvements, list-stores, empty-store, require --access vercel#15147
• Dashboard create modal already defaults to private
• These examples are referenced from the Vercel dashboard (front/apps/vercel-site/components/stores/constants.ts)