Refresh release docs and admin screenshots

[windischb]

Summary

• align README, quickstart, deployment, realm and API docs with the zero-realm first-installation flow
• document adminless realm creation and the separate single-active realm-admin invitation
• replace 1x1 screenshot placeholders with 11 current, tightly cropped admin modal screenshots
• remove stale system-realm and tenant-fallback guidance, plus the obsolete OAuth API wizard comment

Validation

• public VitePress build: 88 pages, llms.txt and llms-full.txt
• in-app VitePress build with /docs/ base
• all referenced screenshots exist, are larger than 1x1 and were visually checked
• git diff --check

The only build warning is the existing Rollup chunk-size warning.