Fix double-space period, shift double-tap; settings; letters from system keymap

- Double-space replaced the space via a Backspace keysym, which clients
  apply after the commit: use delete_surrounding_text with the commit
- Tests use a shared FakeEngine that applies ops to a simulated field
- Double-tap shift locks at a sentence start; typing between taps doesn't
- Organization name set: config lives in ~/.config/sloposk/
- Settings (auto-capitalization, double-space period) with a ⚙ panel
- Letter rows come from the compositor's XKB keymap; other letters of the
  layout become long-press alternates; globe switches the system layout
  over org.kde.keyboard
- One JSON template for non-letter layers; stretch keys even out rows

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

src/app/SystemLayouts.cpp

@@ -0,0 +1,49 @@
+#include "SystemLayouts.h"
+
+#include <QDBusConnection>
+#include <QDBusMessage>
+#include <QDBusReply>
+
+namespace {
+const QString Service = QStringLiteral("org.kde.keyboard");
+const QString Path = QStringLiteral("/Layouts");
+const QString Interface = QStringLiteral("org.kde.KeyboardLayouts");
+
+QDBusMessage call(const QString &method, const QVariantList &args = {})
+{
+    QDBusMessage message = QDBusMessage::createMethodCall(Service, Path, Interface, method);
+    message.setArguments(args);
+    return message;
+}
+} // namespace
+
+SystemLayouts::SystemLayouts(QObject *parent)
+    : QObject(parent)
+{
+    QDBusConnection bus = QDBusConnection::sessionBus();
+    const QDBusReply<uint> reply = bus.call(call(QStringLiteral("getLayout")));
+    m_system = reply.isValid();
+    if (!m_system)
+        return;
+    m_current = reply.value();
+    bus.connect(Service, Path, Interface, QStringLiteral("layoutChanged"), this, SLOT(setCurrent(uint)));
+}
+
+void SystemLayouts::next(quint32 count)
+{
+    if (count < 2)
+        return;
+    const quint32 index = (m_current + 1) % count;
+    if (m_system)
+        QDBusConnection::sessionBus().asyncCall(call(QStringLiteral("setLayout"), {index})); // echoes layoutChanged
+    else
+        setCurrent(index);
+}
+
+void SystemLayouts::setCurrent(uint index)
+{
+    if (index == m_current)
+        return;
+    m_current = index;
+    Q_EMIT currentChanged();
+}

src/app/SystemLayouts.h

@@ -0,0 +1,28 @@
+#pragma once
+
+#include <QObject>
+
+// Which of the system's keyboard layouts is active, followed and switched
+// over KWin's org.kde.keyboard D-Bus interface, so the on-screen and the
+// physical keyboard always agree. Without that service it switches locally.
+class SystemLayouts : public QObject
+{
+    Q_OBJECT
+
+public:
+    explicit SystemLayouts(QObject *parent = nullptr);
+
+    quint32 current() const { return m_current; }
+    // Switches to the next of `count` layouts.
+    void next(quint32 count);
+
+Q_SIGNALS:
+    void currentChanged();
+
+private Q_SLOTS:
+    void setCurrent(uint index);
+
+private:
+    bool m_system = false;
+    quint32 m_current = 0;
+};