Write a form-error page's status only after it renders (closes #128)

Six form-error paths (the login error and the webhook form's validation and name-taken branches) called WriteHeader before rendering, so if the form page's own template failed, the answer kept its 400 or 409 status with an error body instead of a 500. The new renderTemplateStatus renders into the buffer and writes the status only after the page has rendered; renderTemplate sends 200 through it, and those handlers pass their status to it. No handler calls WriteHeader before a render. Login-form tests show the 400 page still renders in full and a failing template answers 500.

Model: opus-5-5

internal/handlers/auth_test.go

@@ -3,6 +3,7 @@ package handlers_test
 import (
 	"context"
 	"fmt"
+	"html/template"
 	"net/http"
 	"net/http/httptest"
 	"net/url"
@@ -404,6 +405,60 @@ func TestLogin_MissingCredentialsRejectedBeforeAnyHash(t *testing.T) {
 	)
 }
 
+// TestLogin_FormErrorAnswersItsStatusWithThePage proves that the login
+// form shown again with an error still answers 400 with the whole page.
+func TestLogin_FormErrorAnswersItsStatusWithThePage(t *testing.T) {
+	t.Parallel()
+
+	var h *handlers.Handlers
+
+	app := newTestApp(t, &h)
+	app.RequireStart()
+
+	t.Cleanup(app.RequireStop)
+
+	w := submitLogin(h, sharedProxyPeer, "", "")
+
+	assert.Equal(t, http.StatusBadRequest, w.Code)
+	assert.Contains(
+		t, w.Body.String(), "Username and password are required",
+	)
+	assert.Contains(
+		t, w.Body.String(), "</html>",
+		"the page must render to completion",
+	)
+}
+
+// TestLogin_FormErrorRenderFailureAnswers500 proves that a login form
+// error page whose template fails answers 500 with the error page and
+// none of the form page, rather than the 400 it meant to send.
+func TestLogin_FormErrorRenderFailureAnswers500(t *testing.T) {
+	t.Parallel()
+
+	var h *handlers.Handlers
+
+	app := newTestApp(t, &h)
+	app.RequireStart()
+
+	t.Cleanup(app.RequireStop)
+
+	// The page prints its error message and then fails.
+	h.AddTemplateForTest("login.html", template.Must(
+		template.New("login").Funcs(template.FuncMap{
+			"fail": func() (string, error) { return "", errMidRender },
+		}).Parse(`{{.Error}}{{fail}}`),
+	))
+
+	w := submitLogin(h, sharedProxyPeer, "", "")
+
+	assert.Equal(t, http.StatusInternalServerError, w.Code)
+	assert.NotContains(
+		t, w.Body.String(), "Username and password are required",
+		"the response must carry no part of the aborted page",
+	)
+	assert.Contains(t, w.Body.String(), "500 Internal Server Error")
+}
+
 // TestLogin_SuccessCreatesSession is the control for the tests above:
 // the success path they assert on really does authenticate.
 func TestLogin_SuccessCreatesSession(t *testing.T) {