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/Theme.qml

@@ -15,6 +15,7 @@ QtObject {
     readonly property color handle: dark ? "#8e8e93" : "#7c7c80"
     readonly property color accent: dark ? "#0a84ff" : "#007aff"
     readonly property color accentText: "#ffffff"
+    readonly property color toggleOn: dark ? "#30d158" : "#34c759"
 
     readonly property real windowRadius: 14
     readonly property real keyRadius: 6