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/channels_screen.dart

@@ -44,11 +44,9 @@ class _ChannelsScreenState extends State<ChannelsScreen>
     with DisconnectNavigationMixin {
   final TextEditingController _searchController = TextEditingController();
   final CommunityStore _communityStore = CommunityStore();
-  Timer? _searchDebounce;
+  final CommunityPskIndex _communityIndex = CommunityPskIndex();
   List<Community> _communities = [];
-
-  // Cache of PSK hex -> Community for quick lookup
-  final Map<String, Community> _pskToCommunity = {};
+  Timer? _searchDebounce;
 
   ChannelMessageStore get _channelMessageStore => ChannelMessageStore();
 
@@ -71,37 +69,11 @@ class _ChannelsScreenState extends State<ChannelsScreen>
     if (mounted) {
       setState(() {
         _communities = communities;
-        _buildPskCommunityMap();
+        _communityIndex.initialize(communities);
       });
     }
   }
 
-  void _buildPskCommunityMap() {
-    _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];
-  }
-
-  /// Returns true if this is the community's public channel
-  bool _isCommunityPublicChannel(Channel channel, Community community) {
-    final publicPsk = community.deriveCommunityPublicPsk();
-    return channel.pskHex == Channel.formatPskHex(publicPsk);
-  }
-
   @override
   void dispose() {
     _searchDebounce?.cancel();
@@ -375,37 +347,37 @@ class _ChannelsScreenState extends State<ChannelsScreen>
     int? dragIndex,
   }) {
     final unreadCount = connector.getUnreadCountForChannel(channel);
-    final community = _getCommunityForChannel(channel);
-    final isCommunityChannel = community != null;
-    final isCommunityPublic =
-        isCommunityChannel && _isCommunityPublicChannel(channel, community);
 
     // Determine icon and colors based on channel type
     IconData icon;
     Color iconColor;
     Color bgColor;
-
-    if (isCommunityChannel) {
-      // Community channel styling
-      iconColor = Colors.purple;
-      bgColor = Colors.purple.withValues(alpha: 0.2);
-      if (isCommunityPublic) {
+    final ChannelType channelType = Channel.getChannelType(
+      channel,
+      _communityIndex,
+    );
+    final bool isCommunityChannel = Channel.isCommunityChannel(channelType);
+    switch (channelType) {
+      case ChannelType.communityPublic:
         icon = Icons.groups;
-      } else {
+        iconColor = Colors.purple;
+        bgColor = Colors.purple.withValues(alpha: 0.2);
+      case ChannelType.communityHashtag:
         icon = Icons.tag;
-      }
-    } else if (channel.isPublicChannel) {
-      icon = Icons.public;
-      iconColor = Colors.green;
-      bgColor = Colors.green.withValues(alpha: 0.2);
-    } else if (channel.name.startsWith('#')) {
-      icon = Icons.tag;
-      iconColor = Colors.blue;
-      bgColor = Colors.blue.withValues(alpha: 0.2);
-    } else {
-      icon = Icons.lock;
-      iconColor = Colors.blue;
-      bgColor = Colors.blue.withValues(alpha: 0.2);
+        iconColor = Colors.purple;
+        bgColor = Colors.purple.withValues(alpha: 0.2);
+      case ChannelType.public:
+        icon = Icons.public;
+        iconColor = Colors.green;
+        bgColor = Colors.green.withValues(alpha: 0.2);
+      case ChannelType.hashtag:
+        icon = Icons.tag;
+        iconColor = Colors.blue;
+        bgColor = Colors.blue.withValues(alpha: 0.2);
+      case ChannelType.private:
+        icon = Icons.lock;
+        iconColor = Colors.blue;
+        bgColor = Colors.blue.withValues(alpha: 0.2);
     }
 
     return Card(