fix: Defer low-power mode during section indexing and book loading

Prevent the device from dropping to 10MHz CPU during first-time chapter
indexing, cover prerendering, and other CPU-intensive reader operations.

Three issues addressed:
- ActivityWithSubactivity now delegates preventAutoSleep() and
  skipLoopDelay() to the active subactivity, so EpubReaderActivity's
  signal is visible through the ReaderActivity wrapper
- Added post-loop() re-check of preventAutoSleep() in main.cpp to
  catch activity transitions that happen mid-loop
- EpubReaderActivity uses both !section and a loadingSection flag to
  cover the full duration from activity entry through section file
  creation; TxtReaderActivity uses !initialized similarly

Also syncs HalPowerManager.cpp log messages with upstream PR #852.

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/hal/HalPowerManager.cpp

@@ -17,14 +17,14 @@ void HalPowerManager::setPowerSaving(bool enabled) {
   if (enabled && !isLowPower) {
     LOG_DBG("PWR", "Going to low-power mode");
     if (!setCpuFrequencyMhz(LOW_POWER_FREQ)) {
-      LOG_ERR("PWR", "Failed to set low-power CPU frequency");
+      LOG_DBG("PWR", "Failed to set CPU frequency = %d MHz", LOW_POWER_FREQ);
       return;
     }
   }
   if (!enabled && isLowPower) {
     LOG_DBG("PWR", "Restoring normal CPU frequency");
     if (!setCpuFrequencyMhz(normalFreq)) {
-      LOG_ERR("PWR", "Failed to restore normal CPU frequency");
+      LOG_DBG("PWR", "Failed to set CPU frequency = %d MHz", normalFreq);
       return;
     }
   }