fix: acquire power lock before sleeping (#1125)

## Summary

Ref: https://github.com/crosspoint-reader/crosspoint-reader/issues/1110

Power lock is automatically acquired on `render()`. However, instead of
using `render()`, sleep activity render everything right inside
`onEnter()`, so no power lock was acquired.

After https://github.com/crosspoint-reader/crosspoint-reader/pull/1016 ,
the power lock will also be acquired on activity transition.

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src/main.cpp

@@ -200,6 +200,7 @@ void waitForPowerRelease() {
 
 // Enter deep sleep mode
 void enterDeepSleep() {
+  HalPowerManager::Lock powerLock;  // Ensure we are at normal CPU frequency for sleep preparation
   APP_STATE.lastSleepFromReader = currentActivity && currentActivity->isReaderActivity();
   APP_STATE.saveToFile();
   exitActivity();