chore: add Ecash App and Harbor to design guide (#1199)

* chore: add Ecash App and Harbor to design guide

* Update guide/how-it-works/ecash/fedimint.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Christoph Ono <chri@sto.ph>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: 3 people

guide/how-it-works/ecash/fedimint.md

@@ -119,6 +119,8 @@ Use Fedimint whenever a custodial Lightning and ecash wallet is required, and si
 ### Wallets
 - [Fedi](https://www.fedi.xyz)
 - [Webimint](https://github.com/elsirion/webimint-rs)
+- [Ecash App](https://ecash.love)
+- [Harbor](https://harbor.cash)
 
 ### Services
 - [Clovyr](https://clovyr.app/)