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.h

@@ -14,3 +14,6 @@ struct KeyboardMetrics {
 // order (spacers included). One key unit has the same width in every row, so
 // rows with equal unit totals line up; shorter rows are centered.
 QList<QRectF> computeKeyRects(const Layer &layer, QSizeF size, KeyboardMetrics metrics = {});
+
+// Total width of a row in key units.
+qreal rowUnits(const Row &row);