fix: properly implement requestUpdateAndWait() (#1218)

## Summary

Properly implement `requestUpdateAndWait()` using freeRTOS direct task
notification.

FWIW, I think most of the current use cases of `requestUpdateAndWait()`
are redundant, better to be replaced by `requestUpdate(true)`. But just
keeping them in case we can find a proper use case for it in the future.

---

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Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

src/activities/ActivityManager.h

@@ -50,6 +50,10 @@ class ActivityManager {
   static void renderTaskTrampoline(void* param);
   [[noreturn]] virtual void renderTaskLoop();
 
+  // Set by requestUpdateAndWait(); read and cleared by the render task after render completes.
+  // Note: only one waiting task is supported at a time
+  TaskHandle_t waitingTaskHandle = nullptr;
+
   // Mutex to protect rendering operations from race conditions
   // Must only be used via RenderLock
   SemaphoreHandle_t renderingMutex = nullptr;
@@ -98,6 +102,10 @@ class ActivityManager {
   // If immediate is true, the update will be triggered immediately.
   // Otherwise, it will be deferred until the end of the current loop iteration.
   void requestUpdate(bool immediate = false);
+
+  // Trigger a render and block until it completes.
+  // Must NOT be called from the render task or while holding a RenderLock.
+  void requestUpdateAndWait();
 };
 
 extern ActivityManager activityManager;  // singleton, to be defined in main.cpp