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: the status, one fixed line explaining it, and a link back to the webhook list, or to sign-in when nobody is signed in. Unknown paths reach it through the router's not-found handler, a refused form token through the CSRF middleware, and a panic through a recoverer each admin page route group installs first. Status codes are unchanged, and the page always sends Cache-Control: no-store.

If the error page fails to render, the answer is the same status in plain text; if it panics, the answer is a 500. The receiver, the healthcheck and /metrics keep their plain answers.

Model: opus-5-5

internal/handlers/handlers_test.go

@@ -307,10 +307,14 @@ func TestRenderTemplateMidRenderErrorSendsNoPartialBody(t *testing.T) {
 		t, http.StatusInternalServerError, w.Code,
 		"a failed render must report a 500",
 	)
-	assert.Equal(
-		t, "Internal server error\n", w.Body.String(),
+	assert.NotContains(
+		t, w.Body.String(), partialPageMarker,
 		"the response must carry no part of the aborted page",
 	)
+	assert.Contains(
+		t, w.Body.String(), "500 Internal Server Error",
+		"a failed render must answer with the error page",
+	)
 }
 
 func TestBuildDatabaseTargetConfig_Valid(t *testing.T) {