Name it wg-app-link

`wg-link` was not merely vague. `wg` and `ip link` are both real network
tools, so it reads as something that manages a WireGuard interface --
misleading rather than ambiguous. `app` rules that reading out, and says
which two ends the link is between.

The repository is still called wg-server-app until it is renamed on
gitea; the crate leads.

server/Cargo.lock

@@ -297,7 +297,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e6e4313cd5fcd3dad5cafa179702e2b244f760991f45397d14d4ebf38247da75"
 
 [[package]]
-name = "wg-link"
+name = "wg-app-link"
 version = "0.1.0"
 dependencies = [
  "anyhow",