Use correct channel icons in channel chat screen

At the top of the channel chat screen is an icon, indicating the
channel type.

Previously, the public icon was used correctly, but the
hashtag icon was used for all other types.

Now, consistent with the channels screen, we use the lock icon for
private channels, and the composite icons for community public &
community hashtag types.

The fix for private channels was trivial, as we can identify hashtag
channels by their name. Finding out whether a channel belongs to a
community is much more involved. All the hard-working code was copied
from channels_screen.dart. (I tried refactoring to reduce duplication,
but my results were complex and not worth it.)

Closes #432

lib/models/channel.dart

@@ -4,6 +4,9 @@ import 'dart:typed_data';
 import 'package:crypto/crypto.dart' as crypto;
 
 import '../connector/meshcore_protocol.dart';
+import 'community.dart';
+
+enum ChannelType { public, private, hashtag, communityPublic, communityHashtag }
 
 class Channel {
   final int index;
@@ -111,5 +114,36 @@ class Channel {
     return bytes.map((b) => b.toRadixString(16).padLeft(2, '0')).join();
   }
 
+  static bool isCommunityChannel(ChannelType channelType) {
+    switch (channelType) {
+      case ChannelType.communityPublic:
+      case ChannelType.communityHashtag:
+        return true;
+      case ChannelType.public:
+      case ChannelType.private:
+      case ChannelType.hashtag:
+        return false;
+    }
+  }
+
+  static ChannelType getChannelType(
+    Channel channel,
+    CommunityPskIndex communityIndex,
+  ) {
+    Community? community = communityIndex.getCommunityForChannel(channel);
+    if (community != null) {
+      if (Community.isCommunityPublicChannel(channel, community)) {
+        return ChannelType.communityPublic;
+      }
+      return ChannelType.communityHashtag;
+    }
+    if (channel.isPublicChannel) {
+      return ChannelType.public;
+    } else if (channel.name.startsWith('#')) {
+      return ChannelType.hashtag;
+    }
+    return ChannelType.private;
+  }
+
   static const String publicChannelPsk = '8b3387e9c5cdea6ac9e5edbaa115cd72';
 }

lib/models/community.dart

@@ -4,6 +4,8 @@ import 'dart:typed_data';
 
 import 'package:crypto/crypto.dart' as crypto;
 
+import 'channel.dart';
+
 /// Represents a community with a shared secret for deriving channel PSKs.
 ///
 /// A Community is a namespace with a shared secret K (32 random bytes),
@@ -162,6 +164,12 @@ class Community {
     return hashtag.replaceFirst(RegExp(r'^#'), '').toLowerCase().trim();
   }
 
+  /// Returns true if this is the community's public channel
+  static bool isCommunityPublicChannel(Channel channel, Community community) {
+    final publicPsk = community.deriveCommunityPublicPsk();
+    return channel.pskHex == Channel.formatPskHex(publicPsk);
+  }
+
   /// Add a hashtag channel to this community's list
   Community addHashtagChannel(String hashtag) {
     final normalized = _normalizeCommunityHashtag(hashtag);
@@ -237,3 +245,28 @@ class Community {
   @override
   int get hashCode => id.hashCode;
 }
+
+class CommunityPskIndex {
+  // Cache of PSK hex -> Community for quick lookup
+  final Map<String, Community> _pskToCommunity = {};
+
+  void initialize(List<Community> communities) {
+    _pskToCommunity.clear();
+    for (final community in communities) {
+      // Map the community public channel PSK
+      final publicPsk = community.deriveCommunityPublicPsk();
+      _pskToCommunity[Channel.formatPskHex(publicPsk)] = community;
+
+      // Map all known hashtag channel PSKs
+      for (final hashtag in community.hashtagChannels) {
+        final hashtagPsk = community.deriveCommunityHashtagPsk(hashtag);
+        _pskToCommunity[Channel.formatPskHex(hashtagPsk)] = community;
+      }
+    }
+  }
+
+  /// Returns the community this channel belongs to, or null if not a community channel
+  Community? getCommunityForChannel(Channel channel) {
+    return _pskToCommunity[channel.pskHex];
+  }
+}