Adds a new home-manager module `ntfy-notify` that:
- Installs the `ntfy` package
- Creates `~/.config/ntfy/client.yml` with subscription config
- Sets up a `systemd --user` service (`ntfy-notify.service`) that
subscribes to ntfy topics and fires `notify-send` on each message
- Subscribes to the `joao-kanban` topic by default
Also wires the module into `hosts/jpporta-nixos/home.nix` with
`custom.ntfy-notify.enable = true`.
