Repeat Slack installs and improved workspace UI

Slack workspace installs can now be repeated: after a workspace is disconnected, re-connecting it no longer fails. The OAuth flow also uses a time-bound, signed state token tied to your account, so install links are safer and no longer rely on session state. Along with this, the account Slack page has been redesigned with clearer workspace cards, status badges for connected or disconnected workspaces, and responsive layout adjustments for narrower screens.