Hold Keys mode: keys stay down while touched, like a hardware keyboard

- HardwareKeys presses a key's unshifted keysym on touch and releases it
  on lift, through fake input; held keys repeat and combine as on a
  physical keyboard (games, Shift+click)
- Shift/Ctrl/Alt/Super are plain held keys, caps lock is the system's,
  dead keys compose in the app
- No long-press, flick, trackpad or rollover in this mode; nothing is
  left held when the mode is turned off or touches are cancelled
- Settings switch; default stays the touch (virtual keyboard) mode

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

qml/SettingsPanel.qml

@@ -32,6 +32,14 @@ Rectangle {
                 onToggled: checked => panel.settings.autoCapitalize = checked
             }
 
+            ToggleRow {
+                width: parent.width
+                text: qsTr("Hold Keys")
+                detail: qsTr("Keys stay down while touched, like a hardware keyboard; no accents or flicks")
+                checked: panel.settings.holdKeys
+                onToggled: checked => panel.settings.holdKeys = checked
+            }
+
             ToggleRow {
                 width: parent.width
                 text: qsTr("Always Show")