Long-press popups: one owner, always on screen

- A second long-press finishes the first (typing its choice) before
  opening its own popup, instead of sharing one
- Popups move down over their key when the room above is too small
- No popup over an unknown key: an empty rect made every cell zero wide
- A repeated touch id cancels the stale pointer instead of leaking it

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

qml/Main.qml

@@ -50,6 +50,7 @@ Window {
             visible: !root.settingsOpen
             model: root.layoutModel
             controller: root.controller
+            popupHeadroom: root.popupMargin + anchors.margins
         }
 
         SettingsPanel {