fix(graphics): make on-screen keyboard lifecycle safe and RAII-managed (#11460)

- VirtualKeyboard::handleLongPress VK_ESC invoked the onTextEntered
  member std::function directly, but that callback path reaches
  OnScreenKeyboardModule::stop(), which destroys the keyboard - and
  with it the std::function whose invocation is still on the stack.
  handlePress and submitText already deliberately copy-and-clear before
  invoking for exactly this reason (CannedMessageModule documents the
  same hazard); do the same here.

- OnScreenKeyboardModule's keyboard becomes unique_ptr, replacing the
  delete-in-destructor / delete-then-new-in-start / delete-in-stop
  bookkeeping that runs on every keyboard open/close. The
  NotificationRenderer legacy hook receives a non-owning raw pointer,
  as before.

src/modules/OnScreenKeyboardModule.h

@@ -7,6 +7,7 @@
 #include "graphics/VirtualKeyboard.h"
 #include <OLEDDisplay.h>
 #include <functional>
+#include <memory>
 #include <string>
 
 namespace graphics
@@ -34,7 +35,7 @@ class OnScreenKeyboardModule
     void onSubmit(const std::string &text);
     void onCancel();
 
-    VirtualKeyboard *keyboard = nullptr;
+    std::unique_ptr<VirtualKeyboard> keyboard;
     std::function<void(const std::string &)> callback;
 };