Add BLE lifecycle pause mechanism for memory mgmt

Introduces a global pause flag to temporarily block BLE stack auto-start during large memory allocations (e.g., in EPUB rendering). Also adds auto-restart logic for BLE scanning when device list is empty, and extensive debug logging for BLE scan lifecycle troubleshooting.

src/activities/ActivityManager.cpp

@@ -254,6 +254,10 @@ bool ActivityManager::isReaderActivity() const {
          (currentActivity && currentActivity->isReaderActivity());
 }
 
+bool ActivityManager::currentKeepsBluetoothAlive() const {
+  return currentActivity && currentActivity->keepsBluetoothAlive();
+}
+
 void ActivityManager::requestGhostCleanup() {
   if (currentActivity) currentActivity->requestGhostCleanup();
 }