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';
 }