Render admin page errors in the normal layout (closes #382)

Every 400, 403, 404 and 500 on an admin page now answers with an
error page in the normal layout: one fixed line for the status and a
link back to the webhook list, or to sign-in when nobody is signed
in. The router's handler for unknown paths and the CSRF middleware's
refusal use the same page. Status codes are unchanged. The receiver,
the healthcheck and /metrics keep their plain answers. If the error
page itself cannot render, the answer is the same status in plain
text.

Model: opus-5-5

internal/handlers/export_test.go

@@ -1,9 +1,11 @@
 package handlers
 
 import (
+	"context"
 	"html/template"
 	"log/slog"
 	"net/http"
+	"net/http/httptest"
 
 	"sneak.berlin/go/webhooker/internal/database"
 )
@@ -63,12 +65,20 @@ func (s *Handlers) LoadEventLogViewsForTest(
 	page int,
 ) []EventLogView {
 	views, _, _ := s.loadEventsWithDeliveries(
-		w, webhook, nil, page,
+		w, newRequestForTest(), webhook, nil, page,
 	)
 
 	return views
 }
 
+// newRequestForTest is the request the helpers here pass on for
+// callers that have none: it is used only to render the error page.
+func newRequestForTest() *http.Request {
+	return httptest.NewRequestWithContext(
+		context.Background(), http.MethodGet, "/", nil,
+	)
+}
+
 // AddTemplateForTest registers a template under a page name so that
 // the handlers_test package can drive the render path with a
 // template of its own.
@@ -122,5 +132,5 @@ func (s *Handlers) BuildDatabaseTargetConfigForTest(
 	w http.ResponseWriter,
 	expiry string,
 ) (string, error) {
-	return s.buildDatabaseTargetConfig(w, expiry)
+	return s.buildDatabaseTargetConfig(w, newRequestForTest(), expiry)
 }