fix: retry characteristic discovery when FFE2 missing after disconnect

After 2+ disconnects during provisioning, the beacon's BLE stack can
return incomplete characteristic discovery results. Instead of hard-
failing with "DX-Smart command characteristic (FFE2) not found", we
now re-trigger characteristic discovery (up to 2 attempts) and resume
writing from the saved position once FFE2 is found.

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

PayfritBeacon/BeaconProvisioner.swift

@@ -190,6 +190,8 @@ class BeaconProvisioner: NSObject, ObservableObject {
     // If a write callback doesn't come back in time, we retry or fail gracefully
     // instead of hanging until the 30s global timeout
     private var writeTimeoutTimer: DispatchWorkItem?
+    private var charRediscoveryCount = 0
+    private static let MAX_CHAR_REDISCOVERY = 2
     private static let WRITE_TIMEOUT_SECONDS: Double = 5.0
     private var writeRetryCount = 0
     private static let MAX_WRITE_RETRIES = 1  // Retry once per command before failing
@@ -336,6 +338,7 @@ class BeaconProvisioner: NSObject, ObservableObject {
         deviceInfoRetryCount = 0
         disconnectRetryCount = 0
         writeRetryCount = 0
+        charRediscoveryCount = 0
         currentBeacon = nil
         state = .idle
         progress = ""
@@ -571,10 +574,29 @@ class BeaconProvisioner: NSObject, ObservableObject {
         progress = "Writing config (\(current)/\(total))..."
 
         guard let commandChar = characteristics[BeaconProvisioner.DXSMART_COMMAND_CHAR] else {
-            fail("DX-Smart command characteristic (FFE2) not found", code: .serviceNotFound)
+            // After disconnect+reconnect, characteristic discovery may return incomplete results.
+            // Re-discover characteristics instead of hard-failing.
+            if charRediscoveryCount < BeaconProvisioner.MAX_CHAR_REDISCOVERY, let service = configService {
+                charRediscoveryCount += 1
+                DebugLog.shared.log("BLE: FFE2 missing — re-discovering characteristics (attempt \(charRediscoveryCount)/\(BeaconProvisioner.MAX_CHAR_REDISCOVERY))")
+                progress = "Re-discovering characteristics..."
+                state = .discoveringServices
+                DispatchQueue.main.asyncAfter(deadline: .now() + 1.0) { [weak self] in
+                    self?.peripheral?.discoverCharacteristics([
+                        BeaconProvisioner.DXSMART_NOTIFY_CHAR,
+                        BeaconProvisioner.DXSMART_COMMAND_CHAR,
+                        BeaconProvisioner.DXSMART_PASSWORD_CHAR
+                    ], for: service)
+                }
+            } else {
+                fail("DX-Smart command characteristic (FFE2) not found after \(charRediscoveryCount) rediscovery attempts", code: .serviceNotFound)
+            }
             return
         }
 
+        // Reset rediscovery counter on successful characteristic access
+        charRediscoveryCount = 0
+
         DebugLog.shared.log("BLE: Writing command \(current)/\(total): \(packet.map { String(format: "%02X", $0) }.joined(separator: " "))")
         writeRetryCount = 0
         scheduleWriteTimeout()
@@ -1294,6 +1316,13 @@ extension BeaconProvisioner: CBPeripheralDelegate {
         if operationMode == .readingConfig {
             // Continue exploring next service
             exploreNextService()
+        } else if charRediscoveryCount > 0 && dxSmartAuthenticated && !dxSmartCommandQueue.isEmpty {
+            // Rediscovery during active write — resume writing directly (already authenticated)
+            DebugLog.shared.log("BLE: Characteristics re-discovered after FFE2 miss — resuming write")
+            state = .writing
+            DispatchQueue.main.asyncAfter(deadline: .now() + 0.5) { [weak self] in
+                self?.dxSmartSendNextCommand()
+            }
         } else {
             // Provisioning: DX-Smart auth flow
             dxSmartStartAuth()