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>

tests/FakeEngine.h

@@ -0,0 +1,34 @@
+#pragma once
+
+#include "engine/InputEngine.h"
+
+#include <QList>
+#include <QString>
+
+// Records what the keyboard sends and applies it to a simulated field, so
+// tests check the text a user would see rather than the calls made.
+class FakeEngine : public InputEngine
+{
+public:
+    void commitText(const QString &text) override { output += text; }
+
+    void replaceBeforeCursor(const QString &before, const QString &text) override
+    {
+        if (output.endsWith(before))
+            output.chop(before.size());
+        else
+            mismatchedReplacements++;
+        output += text;
+    }
+
+    void sendKeysym(quint32 keysym, Qt::KeyboardModifiers modifiers) override
+    {
+        keysyms.append(keysym);
+        lastModifiers = modifiers;
+    }
+
+    QString output;               // the field's text
+    int mismatchedReplacements = 0; // replaced text that wasn't before the cursor
+    QList<quint32> keysyms;
+    Qt::KeyboardModifiers lastModifiers;
+};