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>

qml/SectionHeader.qml

@@ -0,0 +1,9 @@
+import QtQuick
+
+// Small caps-style title above a group of settings.
+Text {
+    color: Theme.secondaryText
+    font.pixelSize: 12
+    font.capitalization: Font.AllUppercase
+    bottomPadding: 4
+}