Keep the window alive on close; add a system toggle to VisibilityController

- Main.qml: ignore close requests (Alt+F4, task manager), so the window
  can't be hidden without VisibilityController knowing.
- main.cpp: don't quit when the last window closes; KWin only restarts
  the input method after a crash.
- VisibilityController::setEnabled(): when false, the keyboard is hidden
  regardless of other state; changing it clears show()/hide() overrides.
  Not wired to KWin's tray toggle yet.

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

qml/Main.qml

@@ -36,6 +36,9 @@ Window {
     visible: false
     color: "transparent"
 
+    // Alt+F4 or the task manager would hide it behind the visibility
+    // controller's back; the dismiss key and the tray toggle hide it instead.
+    onClosing: close => close.accepted = false
     onInputRegionChanged: placement?.setInputRegion(inputRegion)
     onBlurRegionChanged: placement?.setBlurRegion(blurRegion, Theme.windowRadius)
     // For CollapseController.