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/KeyGeometry.cpp

@@ -2,8 +2,6 @@
 
 #include <algorithm>
 
-namespace {
-
 qreal rowUnits(const Row &row)
 {
     qreal units = 0;
@@ -12,8 +10,6 @@ qreal rowUnits(const Row &row)
     return units;
 }
 
-} // namespace
-
 QList<QRectF> computeKeyRects(const Layer &layer, QSizeF size, KeyboardMetrics metrics)
 {
     QList<QRectF> rects;