v1.0.0: feature: rebrand to xui-im (XUI Improved)

Rename project from 3x-ui to xui-im across codebase and app.

- Update Go module and imports to github.com/codewithtamim/xui-im/v2
- Point install.sh, update.sh, x-ui.sh at codewithtamim/xui-im
- Set Docker images to c0dewithtamim/xui-im and ghcr.io/codewithtamim/xui-im
- Rebrand app name 3X-UI -> XUI-IM in config, translations, UI, Swagger, workflows
- Add xui-im banner images and update all READMEs
- Rename docker-compose service to xui-im
- Remove personal/production usage disclaimer from READMEs