feat: migrate BLE to NimBLE — 45% flash, 33% RAM reduction

- Replaced ESP32 BLE Arduino with h2zero/NimBLE-Arduino@^2
- -DCONFIG_BT_NIMBLE_ENABLED to disable built-in Bluedroid
- NIMBLE_PROPERTY::* replaces BLECharacteristic::PROPERTY_* + setAccessPermissions
- BLE2902 descriptor removed (NimBLE auto-creates it)
- pService->start() removed (NimBLE starts services with server)
- setMinPreferred → setMinInterval/setMaxInterval
- onConnect/onDisconnect/onWrite get extra NimBLEConnInfo& param
- Re-advertise: BLEDevice → NimBLEDevice::startAdvertising()
- Firmware: 533KB flash (was 964KB), 29KB RAM (was 44KB)

platformio.ini

@@ -10,6 +10,8 @@ build_flags =
     -DCORE_DEBUG_LEVEL=5
     -DARDUINO_USB_CDC_ON_BOOT=1
     -DARDUINO_USB_MODE=1
+    -DCONFIG_BT_NIMBLE_ENABLED
 lib_deps =
+    h2zero/NimBLE-Arduino@^2
 upload_port = /dev/ttyACM0
 upload_speed = 115200