fix(ios): wire Wi-Fi config characteristic + actually push creds on Confirm

[Flo5k5]

Two stub-level gaps in the open-source app prevented the pairing from
ever completing:

1. AppState.swift never maps the E2E5E5E3 (Config) characteristic to
   BlufiUtil.writeWifiSetCharacteristic. Without the mapping, any
   attempt to sendWifiSetData() early-returns with "No writable
   characteristic".

2. BindingDevice.confirmWifi() only flipped the pairingStatus to
   DistributionNetwork and then ChangeTheName after a 1s delay,
   without ever writing anything over BLE. The firmware never
   received the Wi-Fi credentials, the StackChan stayed offline.

Fix:

• Assign writeWifiSetCharacteristic when the E2E5E5E3 characteristic
  is discovered during the GATT service scan.
• Serialise {"ssid":..., "password":...} as JSON in confirmWifi and
  send it via BlufiUtil.sendWifiSetData before animating the UI
  progression. The firmware handler (hal_ble.cpp:
  _handle_ble_config_write) parses this exact shape.

Both fixes live on self-host for now; they should also land upstream.