chore(release): bump version to 4.0.0

- package.json: 4.0.0-rc7 -> 4.0.0
- package-lock.json refreshed
- src/scss/adminlte.scss: banner version updated
- src/ts/adminlte.ts: banner version updated
- README.md: rewrote the "What's New" section to summarise the v4.0.0
  stable release (18 new pages, docs overhaul, six issue fixes, major
  dep upgrades, breaking changes from v3)
- CHANGELOG.md: marked the unreleased section as [4.0.0] - 2026-05-19
- Docs (introduction / getting-started / deployment): swapped
  @4.0.0-rc7 CDN URLs for @4.0.0