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/network/WifiSelectionActivity.cpp

@@ -244,9 +244,10 @@ void WifiSelectionActivity::checkConnectionStatus() {
 
     // Save this as the last connected network - SD card operations need lock as
     // we use SPI for both
-    xSemaphoreTake(renderingMutex, portMAX_DELAY);
-    WIFI_STORE.setLastConnectedSsid(selectedSSID);
-    xSemaphoreGive(renderingMutex);
+    {
+      RenderLock lock(*this);
+      WIFI_STORE.setLastConnectedSsid(selectedSSID);
+    }
 
     // If we entered a new password, ask if user wants to save it
     // Otherwise, immediately complete so parent can start web server