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/screens/channel_chat_screen.dart

@@ -8,6 +8,8 @@ import 'package:intl/intl.dart';
 import 'package:provider/provider.dart';
 
 import '../connector/meshcore_connector.dart';
+import '../models/community.dart';
+import '../storage/community_store.dart';
 import '../utils/platform_info.dart';
 import '../helpers/chat_scroll_controller.dart';
 import '../connector/meshcore_protocol.dart';
@@ -56,8 +58,11 @@ class _ChannelChatScreenState extends State<ChannelChatScreen> {
   final ChatScrollController _scrollController = ChatScrollController();
   final FocusNode _textFieldFocusNode = FocusNode();
   ChannelMessage? _replyingToMessage;
+  final CommunityStore _communityStore = CommunityStore();
+  final CommunityPskIndex _communityIndex = CommunityPskIndex();
   final Map<String, GlobalKey> _messageKeys = {};
   bool _isLoadingOlder = false;
+  bool _communitiesLoaded = false;
 
   MeshCoreConnector? _connector;
   DateTime? _lastChannelSendAt;
@@ -81,6 +86,7 @@ class _ChannelChatScreenState extends State<ChannelChatScreen> {
       final idx = widget.channel.index;
       final unread = widget.initialUnreadCount;
       final messages = connector.getChannelMessages(widget.channel);
+      _loadCommunities();
       ChannelMessage? anchor;
       if (unread > 0) {
         anchor = _findOldestUnreadChannelAnchor(messages, unread);
@@ -107,6 +113,19 @@ class _ChannelChatScreenState extends State<ChannelChatScreen> {
     });
   }
 
+  // TODO: Reload communities when returning from another screen
+  Future<void> _loadCommunities() async {
+    final connector = context.read<MeshCoreConnector>();
+    _communityStore.setPublicKeyHex = connector.selfPublicKeyHex;
+    final communities = await _communityStore.loadCommunities();
+    if (mounted) {
+      setState(() {
+        _communityIndex.initialize(communities);
+        _communitiesLoaded = true;
+      });
+    }
+  }
+
   ChannelMessage? _findOldestUnreadChannelAnchor(
     List<ChannelMessage> messages,
     int unreadCount,
@@ -193,16 +212,63 @@ class _ChannelChatScreenState extends State<ChannelChatScreen> {
     );
   }
 
+  Widget _channelIcon(Channel channel) {
+    // Determine icon based on channel type
+    final ChannelType channelType = Channel.getChannelType(
+      channel,
+      _communityIndex,
+    );
+    final bool isCommunityChannel = Channel.isCommunityChannel(channelType);
+    IconData icon;
+    switch (channelType) {
+      case ChannelType.communityPublic:
+        icon = Icons.groups;
+      case ChannelType.communityHashtag:
+        icon = Icons.tag;
+      case ChannelType.public:
+        icon = Icons.public;
+      case ChannelType.hashtag:
+        icon = Icons.tag;
+      case ChannelType.private:
+        icon = Icons.lock;
+    }
+    return Stack(
+      children: [
+        Padding(
+          padding: const EdgeInsets.symmetric(vertical: 3),
+          child: _communitiesLoaded
+              ? Icon(icon, size: 20)
+              : SizedBox.square(dimension: 20),
+        ),
+        if (isCommunityChannel)
+          Positioned(
+            right: 0,
+            bottom: 0,
+            child: Container(
+              width: 12,
+              height: 12,
+              decoration: BoxDecoration(
+                color: Colors.purple,
+                shape: BoxShape.circle,
+                border: Border.all(
+                  color: Theme.of(context).cardColor,
+                  width: 2,
+                ),
+              ),
+              child: const Icon(Icons.people, size: 8, color: Colors.white),
+            ),
+          ),
+      ],
+    );
+  }
+
   @override
   Widget build(BuildContext context) {
     return Scaffold(
       appBar: AppBar(
         title: Row(
           children: [
-            Icon(
-              widget.channel.isPublicChannel ? Icons.public : Icons.tag,
-              size: 20,
-            ),
+            _channelIcon(widget.channel),
             const SizedBox(width: 8),
             Expanded(
               child: Column(