fix(talk): cancel button now stops audio playback

The shell '>' redirect creates ACTION_FILE immediately as an empty file.
The poll loop checked [ -f ] (exists) instead of [ -s ] (non-empty),
so it matched on the first 0.2s iteration, found no 'cancel' string,
and broke out of the loop — leaving mpv running with no listener.

[ -s ] only returns true once notify-send writes 'cancel' to the file
(i.e. when the user actually clicks the Abbrechen action).

pkgs/talk/default.nix

@@ -93,7 +93,7 @@
         $KILL "$NOTIFY_PID" 2>/dev/null || true
         break
       fi
-      if [ -f "$ACTION_FILE" ]; then
+      if [ -s "$ACTION_FILE" ]; then
         if $GREP -q "cancel" "$ACTION_FILE" 2>/dev/null; then
           $KILL "$MPV_PID" 2>/dev/null || true
         fi