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/EpubReaderActivity.cpp

@@ -875,19 +875,11 @@ void EpubReaderActivity::render(RenderLock&& lock) {
         built = buildSection();
         const bool bleOk = bleinput::ensureStarted();
         LOG_INF("ERS", "BLE restart after build: begin=%d", bleOk);
-        if (bleOk) {
-          // Show a connecting popup and give the bonded remote a moment to re-link so the
-          // user isn't left with an unresponsive remote and a silent pause. Bounded: a
-          // remote that needs a button press to re-advertise won't stall reading — it
-          // reconnects in the background after we return. The popup is drawn once (e-ink
-          // holds it) and the page render overwrites it when we resume.
-          GUI.drawPopup(renderer, tr(STR_BT_CONNECTING_POPUP));
-          const unsigned long deadline = millis() + 4000;
-          while (!BleHid.isConnected() && millis() < deadline) {
-            BleHid.poll();
-            delay(50);
-          }
-        }
+        // Hold the "BT Connecting..." popup until the remote re-links, then force the
+        // page render below onto the ghost-cleanup (HALF) path so the popup clears
+        // without ghosting the grayscale page.
+        bleinput::showConnectingUntilLinked(renderer, mappedInput);
+        requestGhostCleanup();
       }
       if (!built) {
         LOG_ERR("ERS", "Failed to persist page data to SD");