feat: Linux BLE pairing support via bluetoothctl

Add Linux BLE pairing helper that drives bluetoothctl for pair/trust/PIN
entry, with Completer-based flow control, explicit retry loop, and named
timeout constants.

- LinuxBlePairingService: pair-and-trust with up to 2 retries
- LinuxBleErrorClassifier: map bluetoothctl stderr to user-facing errors
- Conditional import stub for web builds (dart.library.io gate)
- Scanner screen: PIN dialog integration for Linux pairing flow
- MeshCoreConnector: Linux pairing/recovery/reconnect wiring
- l10n: 4 new pairing keys across all 14 locales
- 12 unit tests (pairing service + error classifier)

lib/l10n/app_localizations.dart

@@ -592,6 +592,30 @@ abstract class AppLocalizations {
   /// **'Connection failed: {error}'**
   String scanner_connectionFailed(String error);
 
+  /// No description provided for @scanner_linuxPairingPinTitle.
+  ///
+  /// In en, this message translates to:
+  /// **'Bluetooth Pairing PIN'**
+  String get scanner_linuxPairingPinTitle;
+
+  /// No description provided for @scanner_linuxPairingPinPrompt.
+  ///
+  /// In en, this message translates to:
+  /// **'Enter PIN for {deviceName} (leave blank if none).'**
+  String scanner_linuxPairingPinPrompt(String deviceName);
+
+  /// No description provided for @scanner_linuxPairingShowPin.
+  ///
+  /// In en, this message translates to:
+  /// **'Show PIN'**
+  String get scanner_linuxPairingShowPin;
+
+  /// No description provided for @scanner_linuxPairingHidePin.
+  ///
+  /// In en, this message translates to:
+  /// **'Hide PIN'**
+  String get scanner_linuxPairingHidePin;
+
   /// No description provided for @scanner_stop.
   ///
   /// In en, this message translates to: