Drop unused dependency on `activemodel`

Use × as close button instead of letter X

When the server, e.g., Puma binds to unspecified IPv6 address `[::]`
with ,e.g.,:
```ruby, config/puma.rb
port ENV.fetch("PORT", 3000), "::"
```
the server sees the local client connecting from an IPv4-mapped address
`::ffff:127.0.0.1`. This makes connection to the web console rejected
with:
```
Cannot render console from ::ffff:127.0.0.1! Allowed networks: 127.0.0.0/127.255.255.255, ::1
```
unless loopback addresses expressed as IPv4-mapped addressses are
premitted.

Always permit IPv4-mapped IPv6 loopback addresses

Fix CI

Fix compatiblity with latest rails