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/BleInput.h

@@ -17,6 +17,9 @@
 
 #include <cstdint>
 
+class GfxRenderer;
+class MappedInputManager;
+
 namespace bleinput {
 
 // Advertised central name shown to peripherals during pairing.
@@ -38,4 +41,9 @@ bool encodeKey(const freeink::KeyEvent& ev, uint8_t& kind, uint8_t& value);
 // Writes a null-terminated string into out (e.g. "Page Down", "Key 0x4B").
 void describeKey(uint8_t kind, uint8_t value, char* out, size_t outLen);
 
+// Draw a "BT Connecting..." popup and pump the BLE host until the bonded remote
+// links, the user presses a button to dismiss, or a timeout. No-op if BLE isn't
+// running or is already connected. The caller must redraw afterward to clear it.
+void showConnectingUntilLinked(GfxRenderer& renderer, MappedInputManager& input);
+
 }  // namespace bleinput