Add keepsBluetoothAlive hook to Activity base class

Introduces a virtual method allowing activities to indicate whether they need the BLE stack to remain active. This enables selective teardown of Bluetooth to free heap memory, with the Bluetooth settings screen being able to override this for pairing and scanning functionality.

src/activities/reader/EpubReaderMenuActivity.cpp

@@ -3,7 +3,6 @@
 #include <GfxRenderer.h>
 #include <I18n.h>
 
-#include "BleInput.h"
 #include "CrossPointSettings.h"
 #include "MappedInputManager.h"
 #include "components/UITheme.h"
@@ -80,13 +79,10 @@ void EpubReaderMenuActivity::loop() {
     }
 
     if (selectedAction == MenuAction::TOGGLE_BLUETOOTH) {
-      // Toggle in place and stay in the menu (no reader re-render needed).
+      // Just flip the preference and stay in the menu. The main-loop lifecycle check
+      // brings the BLE stack up/down to match (and shows the "BT Connecting..." popup),
+      // so start/stop has a single owner.
       SETTINGS.bluetoothEnabled = SETTINGS.bluetoothEnabled ? 0 : 1;
-      if (SETTINGS.bluetoothEnabled) {
-        bleinput::ensureStarted();
-      } else {
-        bleinput::stop();
-      }
       SETTINGS.saveToFile();
       requestUpdate();
       return;