fix: reset passwordIndex in readConfig() to prevent stale auth state

Without this reset, if provision() was called first and incremented
passwordIndex, a subsequent readConfig() call would start at the wrong
password index and potentially skip the correct password entirely.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

PayfritBeacon/BeaconProvisioner.swift

@@ -253,6 +253,7 @@ class BeaconProvisioner: NSObject, ObservableObject {
         self.operationMode = .readingConfig
         self.readCompletion = completion
         self.readResult = BeaconCheckResult()
+        self.passwordIndex = 0
         self.characteristics.removeAll()
         self.dxSmartAuthenticated = false
         self.dxSmartNotifySubscribed = false