Layouts, iPad-style rendering, taps, shift and layers

- JSON layouts (en-US, sv-SE inheriting number/symbol layers via base)
- LayoutModel + KeyGeometry: proportional rows, nearest-key hit testing
- KeyboardSurface: multi-pointer touch/mouse routing to the controller
- KeyboardController + ShiftState: iOS shift/caps-lock, layers, globe, dismiss
- QML view with light/dark iPad palette and drag handle
- DevHooks: SLOPOSK_AUTOTYPE, SLOPOSK_SCREENSHOT for headless checks
- Unit tests for shift, loader, geometry and controller

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/layout/KeyGeometry.h

@@ -0,0 +1,16 @@
+#pragma once
+
+#include "Layout.h"
+
+#include <QRectF>
+#include <QSizeF>
+
+struct KeyboardMetrics {
+    qreal padding = 6; // around the whole keyboard
+    qreal gap = 8;     // between keys, horizontally and vertically
+};
+
+// Lays out a layer's keys to fill `size`. Returns one rect per key, in row
+// 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 = {});