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.toml

@@ -1,5 +1,5 @@
 [package]
-name = "wg-link"
+name = "wg-app-link"
 version = "0.1.0"
 edition = "2024"
 description = "The private link between a phone and a machine you run: WireGuard binding, a self-signed CA the app pins, and QR enrollment of a bearer token."