Optional PC key blocks: function row, navigation, numeric keypad

- Template sections placed above or to the right of the typing keys;
  the user picks any mix in settings, typing keys included
- keysym keys send XKB keysyms on press, optionally repeating
- Geometry lays blocks side by side with one key unit; stretch keys are
  resolved at layout time so composed blocks line up
- Toggling a block resizes the window so keys keep their size

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

src/layout/KeyGeometry.h

@@ -6,14 +6,26 @@
 #include <QSizeF>
 
 struct KeyboardMetrics {
-    qreal padding = 6; // around the whole keyboard
-    qreal gap = 8;     // between keys, horizontally and vertically
+    qreal padding = 6;      // around the whole keyboard
+    qreal gap = 8;          // between keys, horizontally and vertically
+    qreal blockGap = 24;    // between side-by-side blocks (main, navigation, numpad)
 };
 
-// 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 = {});
+// The keys on screen: blocks of rows, side by side.
+using Block = QList<Row>;
 
-// Total width of a row in key units.
+// Lays out blocks to fill `size`. Returns one rect per key, block by block in
+// row order (spacers included). One key unit has the same width everywhere;
+// all rows share one height, and shorter blocks sit at the bottom. In each
+// block, stretch keys grow to make their row as wide as the widest one, and
+// remaining shorter rows are centered.
+QList<QRectF> computeKeyRects(const QList<Block> &blocks, QSizeF size, KeyboardMetrics metrics = {});
+inline QList<QRectF> computeKeyRects(const Layer &layer, QSizeF size, KeyboardMetrics metrics = {})
+{
+    return computeKeyRects({layer.rows}, size, metrics);
+}
+
+// Total width of a row in key units, before stretching.
 qreal rowUnits(const Row &row);
+// Width of all blocks in key units, ignoring gaps.
+qreal totalUnits(const QList<Block> &blocks);