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>

src/layout/LayoutLoader.cpp

@@ -1,6 +1,7 @@
 #include "LayoutLoader.h"
 
 #include <QFile>
+#include <algorithm>
 #include <QJsonArray>
 #include <QJsonDocument>
 #include <QJsonObject>
@@ -48,6 +49,10 @@ std::optional<KeyDef> parseKey(const QJsonValue &value, QString *error)
     KeyDef key;
     if (value.isString()) {
         key.label = value.toString();
+        if (key.label.isEmpty()) {
+            *error = QStringLiteral("empty key \"\"");
+            return std::nullopt;
+        }
         return key;
     }
     if (!value.isObject()) {
@@ -104,6 +109,14 @@ std::optional<KeyDef> parseKey(const QJsonValue &value, QString *error)
         *error = QStringLiteral("modifier must be one of %1").arg(ModifierNames.join(u", "));
         return std::nullopt;
     }
+    if (key.repeat && key.type != KeyType::Keysym) {
+        *error = QStringLiteral("\"%1\": only keysym keys repeat").arg(typeName);
+        return std::nullopt;
+    }
+    if (key.stretch && (key.type == KeyType::Letters || key.type == KeyType::Keys)) {
+        *error = QStringLiteral("\"%1\" rows can't stretch; their keys come from the keymap").arg(typeName);
+        return std::nullopt;
+    }
     if (key.width <= 0) {
         *error = QStringLiteral("key \"%1\" has a non-positive width").arg(key.label);
         return std::nullopt;
@@ -197,6 +210,10 @@ std::optional<Layout> parse(const QByteArray &json, QString *error)
         auto layer = parseLayer(layerValue.toObject(), error);
         if (!layer)
             return std::nullopt;
+        if (layout.layer(layer->name)) {
+            *error = QStringLiteral("duplicate layer \"%1\"").arg(layer->name);
+            return std::nullopt;
+        }
         layout.layers.append(*layer);
     }
 
@@ -204,6 +221,10 @@ std::optional<Layout> parse(const QByteArray &json, QString *error)
         auto section = parseSection(sectionValue.toObject(), error);
         if (!section)
             return std::nullopt;
+        if (std::ranges::any_of(layout.sections, [&](const Section &s) { return s.id == section->id; })) {
+            *error = QStringLiteral("duplicate section \"%1\"").arg(section->id);
+            return std::nullopt;
+        }
         layout.sections.append(*section);
     }