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/graphics/VirtualKeyboard.cpp

@@ -666,7 +666,13 @@ void VirtualKeyboard::handleLongPress()
         break;
     case VK_ESC:
         if (onTextEntered) {
-            onTextEntered("");
+            // Copy-and-clear before invoking, like handlePress/submitText: the callback can
+            // destroy this keyboard (OnScreenKeyboardModule::stop), so the member must not be
+            // the std::function still executing on the stack.
+            std::function<void(const std::string &)> callback = onTextEntered;
+            onTextEntered = nullptr;
+            inputText = "";
+            callback("");
         }
         break;
     default: