feat: Add community management features with QR code scanning

- Implement Community model for managing community data, including secret handling and PSK derivation.
- Create CommunityQrScannerScreen for scanning and joining communities via QR codes.
- Develop CommunityStore for persisting community data using SharedPreferences.
- Introduce QrCodeDisplay widget for displaying QR codes with customizable options.
- Add QrScannerWidget for reusable QR code scanning functionality with validation and controls.

lib/models/channel.dart

@@ -73,6 +73,35 @@ class Channel {
     return Uint8List.fromList(hash.sublist(0, 16));
   }
 
+  /// Derive PSK for community public channel using HMAC-SHA256.
+  /// PSK = HMAC-SHA256(K, "channel:v1:__public__")[:16]
+  ///
+  /// This creates a channel that is "public" only to members who have
+  /// the community secret. Outsiders see only opaque IDs.
+  static Uint8List deriveCommunityPublicPsk(Uint8List secret) {
+    final hmac = crypto.Hmac(crypto.sha256, secret);
+    final digest = hmac.convert(utf8.encode('channel:v1:__public__'));
+    return Uint8List.fromList(digest.bytes.sublist(0, 16));
+  }
+
+  /// Derive PSK for community hashtag channel using HMAC-SHA256.
+  /// PSK = HMAC-SHA256(K, "channel:v1:" + normalized_name)[:16]
+  ///
+  /// Community hashtag channels are deterministic for all members
+  /// (same name => same id) but impossible to enumerate/guess without K.
+  static Uint8List deriveCommunityHashtagPsk(Uint8List secret, String hashtag) {
+    final normalized = _normalizeCommunityHashtag(hashtag);
+    final hmac = crypto.Hmac(crypto.sha256, secret);
+    final digest = hmac.convert(utf8.encode('channel:v1:$normalized'));
+    return Uint8List.fromList(digest.bytes.sublist(0, 16));
+  }
+
+  /// Normalize a hashtag name for consistent community PSK derivation.
+  /// Strips leading #, converts to lowercase, trims whitespace.
+  static String _normalizeCommunityHashtag(String hashtag) {
+    return hashtag.replaceFirst(RegExp(r'^#'), '').toLowerCase().trim();
+  }
+
   static String formatPskHex(Uint8List psk) {
     return _bytesToHex(psk);
   }