stop panning when the view loses focus

Holding A or D while a modal opened - the escape menu, a schematic choice, game
over - left the pan key held forever: the key-up went to the dialog and never
reached the view, so the world panned on its own once the dialog closed. Panning
runs on wall-clock time rather than ticks, so pausing did not mask it either.

Focus loss now drops every held action. This is a behaviour change, not a move,
so it is its own commit; it is only three lines because the input mapper already
owns the held state and can clear all of it at once.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JcReq7hVk4KUPhTDKWAG7K

src/ui/GameWorldView.h

@@ -107,6 +107,10 @@ protected:
     void paintGL() override;
     void keyPressEvent(QKeyEvent* event) override;
     void keyReleaseEvent(QKeyEvent* event) override;
+    // Key-up never arrives for a key that was still held when focus moved away —
+    // to a modal dialog, another widget, or another window. Any held action would
+    // otherwise stay held forever, so focus loss drops all of them.
+    void focusOutEvent(QFocusEvent* event) override;
     void mousePressEvent(QMouseEvent* event) override;
     void mouseMoveEvent(QMouseEvent* event) override;
     void mouseReleaseEvent(QMouseEvent* event) override;