Polish `NUXT_AUTH_ORIGIN` logic

[phoenix-ru]

Follow up on #837

          Does not need to be done in this PR. but as a follow-up let's:

• Adapt the ~/playground-* to use NUXT_AUTH_ORIGIN
• Adapt any starter to use NUXT_AUTH_ORIGIN
• Adapt the nuxt-auth-example to use NUXT_AUTH_ORIGIN

Originally posted by @zoey-kaiser in https://github.com/sidebase/nuxt-auth/pull/837#pullrequestreview-2227333420

Additionally:

• [ ] Change default originEnvKey to NUXT_AUTH_ORIGIN and automatically strip NUXT_ prefix for camel-case config keys
• [ ] Check type of variable from config to be string | undefined - report error otherwise