Reject more template mistakes

Empty keys, duplicate layer names and section ids, repeat on non-keysym
keys and stretch on letters/keys placeholders.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/tst_layoutloader.cpp

@@ -40,6 +40,34 @@ private Q_SLOTS:
         QVERIFY(error.contains(u"warp"));
     }
 
+    void rejectsInvalidTemplates_data()
+    {
+        QTest::addColumn<QByteArray>("json");
+        QTest::addColumn<QString>("why");
+        const auto row = [](const char *key) {
+            return QByteArray(R"({"id": "t", "layers": [{"name": "l", "rows": [[)") + key + "]]}]}";
+        };
+        QTest::newRow("empty key") << row(R"("")") << u"empty"_s;
+        QTest::newRow("repeat") << row(R"({"type": "backspace", "repeat": true})") << u"repeat"_s;
+        QTest::newRow("stretch") << row(R"({"type": "letters", "row": 0, "stretch": true})") << u"stretch"_s;
+        QTest::newRow("layer twice")
+            << QByteArray(R"({"id": "t", "layers": [{"name": "l", "rows": [["a"]]}, {"name": "l", "rows": [["b"]]}]})")
+            << u"duplicate layer"_s;
+        QTest::newRow("section twice")
+            << QByteArray(R"({"id": "t", "layers": [{"name": "l", "rows": [["a"]]}], "sections": [)"
+                          R"({"id": "s", "rows": [["b"]]}, {"id": "s", "rows": [["c"]]}]})")
+            << u"duplicate section"_s;
+    }
+
+    void rejectsInvalidTemplates()
+    {
+        QFETCH(QByteArray, json);
+        QFETCH(QString, why);
+        QString error;
+        QVERIFY(!LayoutLoader::parse(json, &error));
+        QVERIFY2(error.contains(why), qPrintable(error));
+    }
+
     void rejectsMissingId()
     {
         QString error;