Keep blocks still when switching layers

- The main block reserves the width and rows of its biggest layer, so the
  numpad and function row no longer shift between letters and symbols
- Smaller layers stretch into the room: stretch keys widen, rows grow
  taller (rows above the layer, like the function row, keep their height)

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

tests/tst_controller.cpp

@@ -235,6 +235,20 @@ private Q_SLOTS:
         QCOMPARE(model.rowCount(), mainKeys); // never nothing
     }
 
+    void switchingLayersKeepsTheOtherBlocksStill()
+    {
+        model.setLayout(TestLayouts::build("us", KeyboardMode::Full)); // letters have more rows than symbols
+        model.setVisibleSections({u"function"_s, MainSectionId, u"numpad"_s});
+        model.setSize({1000, 300});
+        const auto lastKeyRect = [this] { return model.keyRect(model.rowCount() - 1); }; // numpad "="
+        const auto firstKeyRect = [this] { return model.keyRect(0); };                   // esc
+        const QRectF numpad = lastKeyRect();
+        const QRectF esc = firstKeyRect();
+        model.setLayer(u"numbers"_s);
+        QCOMPARE(lastKeyRect(), numpad);
+        QCOMPARE(firstKeyRect(), esc);
+    }
+
     KeyDef keysymKey(quint32 keysym)
     {
         KeyDef key = makeKey(KeyType::Keysym);