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/server/middleware_internal_test.go

@@ -11,11 +11,12 @@ import (
 )
 
 // The addresses the forwarding tests use: the client a proxy names in
-// X-Forwarded-For, a peer inside the trusted range of
-// TestRealIPMiddlewareTrustsOnlyConfiguredProxies, and a peer outside it,
-// which lies in testPrefixV4 so that an IP lookup of it finds a route.
+// X-Forwarded-For, the range of trusted proxies, a peer inside it, and a peer
+// outside it, which lies in testPrefixV4 so that an IP lookup of it finds a
+// route.
 const (
 	forwardedClient   = "203.0.113.9"
+	trustedProxyRange = "10.0.0.0/8"
 	trustedPeer       = "10.0.0.2:1234"
 	untrustedPeerAddr = "192.0.2.1"
 	untrustedPeer     = untrustedPeerAddr + ":1234"
@@ -189,7 +190,7 @@ func TestForwardedForFromUntrustedPeerIsIgnored(t *testing.T) {
 func TestRealIPMiddlewareTrustsOnlyConfiguredProxies(t *testing.T) {
 	t.Parallel()
 
-	trusted := []netip.Prefix{netip.MustParsePrefix("10.0.0.0/8")}
+	trusted := []netip.Prefix{netip.MustParsePrefix(trustedProxyRange)}
 
 	tests := []struct {
 		name         string