feat!: rename package to @ouim/logto-authkit and /server

[thezem]

Summary

• rename the published package from @ouim/simple-logto to @ouim/logto-authkit
• rename the public server subpath from /backend to /server
• move the internal source tree from src/backend to src/server
• update docs, examples, smoke fixtures, issue templates, and packaging checks to the new package identity
• remove the old-package deprecation helper from the new canonical package

Included

• package.json and package-lock.json updated to @ouim/logto-authkit@0.3.0
• export map changed from ./backend to ./server
• Vite library build changed from dist/backend/* to dist/server/*
• README and server docs updated to the new install/import paths
• smoke fixtures and examples updated to import @ouim/logto-authkit and @ouim/logto-authkit/server
• bundle-size and package-audit expectations updated for dist/server

Validation

• npm run lint
• npx tsc --project tsconfig.build.json --noEmit
• npx vitest run
• npm run build
• npm run test:size
• npm run test:package
• npm run test:smoke

Notes

• repository.url still points at ouim-me/simple-logto intentionally; the repo rename is a later phase.
• The existing v0.2.1 draft release remains the old-name line. This PR is the 0.3.0 rename line.