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>

qml/SettingsButton.qml

@@ -0,0 +1,24 @@
+import QtQuick
+
+// Gear in the grab bar; opens and closes the settings panel.
+Item {
+    id: button
+
+    property bool checked: false
+    signal clicked()
+
+    implicitWidth: 36
+    implicitHeight: 22
+
+    Text {
+        anchors.centerIn: parent
+        text: button.checked ? "⌨" : "⚙"
+        font.pixelSize: 15
+        color: tap.pressed ? Theme.text : Theme.handle
+    }
+
+    TapHandler {
+        id: tap
+        onTapped: button.clicked()
+    }
+}