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/handlers_internal_test.go

@@ -7,6 +7,7 @@ import (
 	"html"
 	"net/http"
 	"net/http/httptest"
+	"net/netip"
 	"runtime"
 	"slices"
 	"strconv"
@@ -503,6 +504,64 @@ func TestPagesSayOriginZeroIsNoKnownOrigin(t *testing.T) {
 	}
 }
 
+// TestIndexShowsTheClientAddress requests the home page and checks that it has
+// the IP information API card, holding the address /ip/ would look up for the
+// same request: the TCP peer, or, when the peer is a trusted proxy, the client
+// the proxy names in X-Forwarded-For
+// (https://git.eeqj.de/sneak/routewatch/issues/156).
+func TestIndexShowsTheClientAddress(t *testing.T) {
+	t.Parallel()
+
+	const cardHeading = "<h2>IP Information API</h2>"
+
+	// Both requests carry an X-Forwarded-For naming forwardedClient; only a
+	// trusted proxy's is believed.
+	tests := []struct {
+		name           string
+		trustedProxies []netip.Prefix
+		peer           string
+		want           string
+	}{
+		{"direct request", nil, untrustedPeer, untrustedPeerAddr},
+		{
+			"request through a trusted proxy",
+			[]netip.Prefix{netip.MustParsePrefix(trustedProxyRange)},
+			trustedPeer, forwardedClient,
+		},
+	}
+
+	for _, tt := range tests {
+		// The home page reads nothing from the database, so the server has none.
+		s := New(nil, streamer.New(logger.New(), metrics.New()), logger.New(),
+			&config.Config{TrustedProxies: tt.trustedProxies})
+
+		req := httptest.NewRequestWithContext(t.Context(), http.MethodGet, "/", nil)
+		req.RemoteAddr = tt.peer
+		req.Header.Set("X-Forwarded-For", forwardedClient)
+
+		rec := httptest.NewRecorder()
+		s.router.ServeHTTP(rec, req)
+
+		if rec.Code != http.StatusOK {
+			t.Errorf("%s: status %d, want %d; body %s",
+				tt.name, rec.Code, http.StatusOK, rec.Body)
+
+			continue
+		}
+
+		page := rec.Body.String()
+
+		if !strings.Contains(page, cardHeading) {
+			t.Errorf("%s: the page has no IP information API card", tt.name)
+		}
+
+		want := `data-client-ip="` + tt.want + `"`
+		if !strings.Contains(page, want) {
+			t.Errorf("%s: the card does not hold %s", tt.name, want)
+		}
+	}
+}
+
 // TestPagesShowAgesInWords stores routes last updated about 5 minutes, 3 hours
 // and 15 days ago, and checks that the AS page and the prefix pages show each
 // age in words. Before the fix the ages read "5m0s ago", "3h0m0s ago" and