fix: use RAII render lock everywhere (#916)

## Summary

Follow-up to
https://github.com/crosspoint-reader/crosspoint-reader/pull/774

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Did you use AI tools to help write this code? **NO**


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

## Release Notes

* **Refactor**
* Modernized internal synchronization mechanisms across multiple
components to improve code reliability and maintainability. All
functionality remains unchanged.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

src/activities/settings/ClearCacheActivity.cpp

@@ -118,9 +118,10 @@ void ClearCacheActivity::loop() {
   if (state == WARNING) {
     if (mappedInput.wasPressed(MappedInputManager::Button::Confirm)) {
       LOG_DBG("CLEAR_CACHE", "User confirmed, starting cache clear");
-      xSemaphoreTake(renderingMutex, portMAX_DELAY);
-      state = CLEARING;
-      xSemaphoreGive(renderingMutex);
+      {
+        RenderLock lock(*this);
+        state = CLEARING;
+      }
       requestUpdateAndWait();
 
       clearCache();