v1.7.0 - Web UI sign-out button + API-key management docs

Added

• Web UI: a Sign out button on the form and result pages that ends the
  server-side session (POST <BasePath>/logout), shown only in
  WebUi.AuthMode = 'WindowsAd' (standalone) mode. New Get-OdjLogoutForm
  builder plus a -ShowLogout switch on Get-OdjFormBody / Get-OdjResultBody,
  wired up from the /ui, provision-error and result render sites.
• tests/Unit/OfflineJoinWebUi.Tests.ps1: coverage for Get-OdjLogoutForm and
  the conditional logout button on the form and result pages.

Changed

• docs/quickstart.md, docs/schnellstart.md: new Managing and rotating API
  keys section (one-way SHA-256 hashing, the caller keeps the clear-text key,
  adding vs. rotating clients, and the warning that re-running install.ps1
  overwrites the whole ApiClients block). Documented the Web UI Sign out
  button in the security model (standalone mode only; IIS/SSO handles sign-out
  otherwise).

Full changelog: v1.6.11...v1.7.0