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>

src/main.cpp

@@ -440,6 +440,13 @@ void loop() {
     }
   }
 
+  // Re-check preventAutoSleep: the activity may have changed during loop() above
+  // (e.g., HomeActivity transitioned to EpubReaderActivity with pending section work).
+  if (currentActivity && currentActivity->preventAutoSleep()) {
+    lastActivityTime = millis();
+    powerManager.setPowerSaving(false);
+  }
+
   // Add delay at the end of the loop to prevent tight spinning
   // When an activity requests skip loop delay (e.g., webserver running), use yield() for faster response
   // Otherwise, use longer delay to save power