Repeated Slack workspace installs work and the workspace UI is polished

The account/Slack workspace UI no longer breaks when installing multiple Slack workspaces into Hive, and it has a refreshed look. Previously the install flow tracked the OAuth state in a plain session value, which prevented repeating the flow cleanly. The state is now a signed token tied to the session and user, so repeated installs work as expected. The workspace list area in the account page also gets a new layout with clearer empty state, installation rows, responsive styling down to small screens, and consistent icon and typography treatment.