Add support for private and hashtag channels in localization and channel management

- Updated Polish, Portuguese, Slovak, Slovenian, Swedish, and Chinese localization files to include new strings for creating and joining private channels, as well as joining hashtag channels.
- Enhanced the channel management UI to allow users to create and join private channels, join public channels, and join channels via hashtags.
- Implemented PSK derivation from hashtags using SHA256 in the Channel model.
- Improved the translation script to handle missing keys and translate all locales efficiently.

lib/models/channel.dart

@@ -1,5 +1,8 @@
+import 'dart:convert';
 import 'dart:typed_data';
 
+import 'package:crypto/crypto.dart' as crypto;
+
 import '../connector/meshcore_protocol.dart';
 
 class Channel {
@@ -61,6 +64,15 @@ class Channel {
     return bytes;
   }
 
+  /// Derive PSK from hashtag name using SHA256.
+  /// The hashtag is normalized to include '#' prefix.
+  /// Returns first 16 bytes of SHA256 hash as PSK.
+  static Uint8List derivePskFromHashtag(String hashtag) {
+    final name = hashtag.startsWith('#') ? hashtag : '#$hashtag';
+    final hash = crypto.sha256.convert(utf8.encode(name)).bytes;
+    return Uint8List.fromList(hash.sublist(0, 16));
+  }
+
   static String formatPskHex(Uint8List psk) {
     return _bytesToHex(psk);
   }