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/SettingsPanel.qml

@@ -0,0 +1,33 @@
+import QtQuick
+import Sloposk
+
+// Replaces the keys while open. Binds straight to Settings, which persists.
+Rectangle {
+    id: panel
+
+    required property Settings settings
+
+    color: Theme.background
+    radius: Theme.windowRadius
+
+    Column {
+        anchors { fill: parent; margins: 16 }
+        spacing: 4
+
+        ToggleRow {
+            width: parent.width
+            text: qsTr("Auto-Capitalization")
+            detail: qsTr("Shift at the start of a sentence")
+            checked: panel.settings.autoCapitalize
+            onToggled: checked => panel.settings.autoCapitalize = checked
+        }
+
+        ToggleRow {
+            width: parent.width
+            text: qsTr("“.” Shortcut")
+            detail: qsTr("Double-tap space for a period and a space")
+            checked: panel.settings.doubleSpacePeriod
+            onToggled: checked => panel.settings.doubleSpacePeriod = checked
+        }
+    }
+}