refactor(pnpr): source --listen default from Config::DEFAULT_LISTEN (#12629)

Address CodeRabbit review: instead of duplicating the default listen
address as a string literal in the CLI arg, reference the existing
Config::DEFAULT_LISTEN constant so the CLI default and the config default
cannot drift apart.

pnpr/crates/pnpr/src/main.rs

@@ -14,7 +14,7 @@ struct Args {
     config: Option<PathBuf>,
 
     /// Address to bind to.
-    #[arg(long, default_value = "127.0.0.1:7677")]
+    #[arg(long, default_value = Config::DEFAULT_LISTEN)]
     listen: SocketAddr,
 
     /// Override the storage path from the loaded config (bundled or