Show the IP information API on the home page (closes #156)

The home page gets an IP information API card. It shows the curl
command that looks up the visitor's own address at /ip/{addr}. The page
then makes that same request and shows the JSON answer, pretty-printed.
An error answer, such as a 404 for an address with no route, is shown
the same way. A request that fails shows an error line.

The server gives the page the visitor's address, found as /ip/ finds
it, so a trusted proxy's X-Forwarded-For is honoured. The command takes
its origin from the browser, so it names the host the visitor used.

Model: opus-5-5

internal/templates/templates_test.go

@@ -10,13 +10,14 @@ import (
 	"git.eeqj.de/sneak/routewatch/internal/templates"
 )
 
-// renderPage renders a page with the data every page needs.
+// renderPage renders a page with the data every page needs, and the visitor's
+// address, which the home page also needs.
 func renderPage(t *testing.T, tmpl *template.Template) string {
 	t.Helper()
 
 	var page bytes.Buffer
 
-	err := tmpl.Execute(&page, struct{ CSPNonce string }{})
+	err := tmpl.Execute(&page, struct{ CSPNonce, ClientIP string }{})
 	if err != nil {
 		t.Fatalf("rendering the page: %v", err)
 	}