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/layout/Layout.h

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <QHash>
 #include <QList>
 #include <QString>
 #include <QStringList>
@@ -14,6 +15,7 @@ enum class KeyType {
     Globe,
     Dismiss,
     Spacer, // empty room, e.g. the half-key indent of the home row
+    Letters, // template only: a row of letters from the system keymap
 };
 
 struct KeyDef {
@@ -24,6 +26,11 @@ struct KeyDef {
     QStringList alternates; // long-press choices
     QString target;         // layer to switch to (LayerSwitch only)
     qreal width = 1.0;      // in key units
+    bool stretch = false;   // grows so the row is as wide as the layer's widest
+
+    // Letters placeholders only.
+    int lettersRow = -1;    // which keymap row (0 = top)
+    QStringList flicks;     // flick symbol per letter, left to right
 
     // Text this key shows/types; only character keys change case.
     QString text(bool uppercase) const
@@ -42,11 +49,11 @@ struct Layer {
 };
 
 struct Layout {
-    QString id;      // e.g. "en-US"
-    QString name;    // human readable
-    QString locale;  // e.g. "en_US"
-    QString base;    // id of a layout to take missing layers from
+    QString id;
+    QString name;        // human readable, e.g. "Slovak"
     QList<Layer> layers; // the first one is shown by default
+    // Template only: long-press choices for keymap letters, by lowercase letter.
+    QHash<QString, QStringList> alternates;
 
     const Layer *layer(const QString &layerName) const
     {