feat(#289): channel list in the nav drawer, switchable from inside a channel

Epic B of the navigation redesign (#102). Fills the drawer Epic A left empty
and puts it where the reported pain actually is: inside an open channel.

Previously, checking another channel meant backing out to the list, scanning
it, and navigating back in, with no way to see per-channel unread counts while
reading a channel. Now the hamburger opens a channel list showing every
channel's unread count, and tapping one switches straight to it.

- New lib/widgets/channel_drawer_list.dart. Per-tile unread via
  context.select on getUnreadCountForChannelIndex, so a new message rebuilds
  only that tile. Reuses UnreadBadge. Highlights the current channel.
- ChannelChatScreen gains the drawer plus an explicit leading hamburger: it is
  a pushed route, so Scaffold would render a back arrow in that slot instead.
  Switching uses pushReplacement, keeping the back stack one deep so system
  back still returns to the channel list (owner decision, #84).
- ChannelsScreen shares one _openChannel path between its tiles and the
  drawer, and closes the drawer before pushing.

flutter analyze clean, dart format applied. Not yet run on hardware.

lib/screens/channel_chat_screen.dart

@@ -28,6 +28,7 @@ import '../services/block_service.dart';
 import '../services/chat_text_scale_service.dart';
 import '../services/translation_service.dart';
 import '../utils/emoji_utils.dart';
+import '../widgets/channel_drawer_list.dart';
 import '../widgets/mention_autocomplete.dart';
 import '../helpers/emoji_shortcodes.dart';
 import '../widgets/chat_zoom_wrapper.dart';
@@ -271,10 +272,49 @@ class _ChannelChatScreenState extends State<ChannelChatScreen> {
     );
   }
 
+  /// Switch to another channel from the drawer without backing out to the
+  /// channel list. pushReplacement keeps the back stack one deep, so system
+  /// back still returns to the channel list rather than the previous channel.
+  void _switchChannel(Channel channel) {
+    if (channel.index == widget.channel.index) {
+      Navigator.pop(context);
+      return;
+    }
+    final connector = context.read<MeshCoreConnector>();
+    final unread = connector.getUnreadCountForChannelIndex(channel.index);
+    connector.markChannelRead(channel.index);
+    Navigator.pushReplacement(
+      context,
+      MaterialPageRoute(
+        builder: (context) =>
+            ChannelChatScreen(channel: channel, initialUnreadCount: unread),
+      ),
+    );
+  }
+
   @override
   Widget build(BuildContext context) {
     return Scaffold(
+      drawer: Drawer(
+        width: 300,
+        child: SafeArea(
+          child: ChannelDrawerList(
+            currentChannelIndex: widget.channel.index,
+            onChannelSelected: _switchChannel,
+          ),
+        ),
+      ),
       appBar: AppBar(
+        // Explicit hamburger: this is a pushed route, so Scaffold would show a
+        // back arrow here instead of the drawer button. System back still pops
+        // to the channel list.
+        leading: Builder(
+          builder: (context) => IconButton(
+            icon: const Icon(Icons.menu),
+            tooltip: context.l10n.channels_title,
+            onPressed: () => Scaffold.of(context).openDrawer(),
+          ),
+        ),
         title: Row(
           children: [
             _channelIcon(widget.channel),

lib/screens/channels_screen.dart

@@ -24,6 +24,7 @@ import '../widgets/list_filter_widget.dart';
 import '../widgets/empty_state.dart';
 import '../widgets/qr_code_display.dart';
 import '../widgets/app_shell.dart';
+import '../widgets/channel_drawer_list.dart';
 import '../widgets/sync_progress_overlay.dart';
 import '../widgets/unread_badge.dart';
 import '../helpers/snack_bar_builder.dart';
@@ -105,6 +106,9 @@ class _ChannelsScreenState extends State<ChannelsScreen>
         onDestinationSelected: (index) => _handleQuickSwitch(index, context),
         contactsUnreadCount: connector.getTotalContactsUnreadCount(),
         channelsUnreadCount: connector.getTotalChannelsUnreadCount(),
+        drawerContent: ChannelDrawerList(
+          onChannelSelected: (channel) => _openChannel(context, channel),
+        ),
         appBar: AppBar(
           title: AppBarTitle(context.l10n.channels_title),
           centerTitle: true,
@@ -629,6 +633,28 @@ class _ChannelsScreenState extends State<ChannelsScreen>
     );
   }
 
+  /// Shared open path for the channel tiles and the nav drawer list.
+  Future<void> _openChannel(BuildContext context, Channel channel) async {
+    final connector = context.read<MeshCoreConnector>();
+    final unread = connector.getUnreadCountForChannelIndex(channel.index);
+    connector.markChannelRead(channel.index);
+    // Close the drawer if the tap came from it, so it is not left open behind
+    // the chat screen when the user comes back.
+    final scaffold = Scaffold.maybeOf(context);
+    if (scaffold?.isDrawerOpen ?? false) {
+      Navigator.pop(context);
+    }
+    await Future.delayed(const Duration(milliseconds: 50));
+    if (!context.mounted) return;
+    Navigator.push(
+      context,
+      MaterialPageRoute(
+        builder: (context) =>
+            ChannelChatScreen(channel: channel, initialUnreadCount: unread),
+      ),
+    );
+  }
+
   void _handleQuickSwitch(int index, BuildContext context) {
     if (index == 1) return;
     switch (index) {

lib/widgets/channel_drawer_list.dart

@@ -0,0 +1,97 @@
+import 'package:flutter/material.dart';
+import 'package:provider/provider.dart';
+
+import '../connector/meshcore_connector.dart';
+import '../l10n/l10n.dart';
+import '../models/channel.dart';
+import 'unread_badge.dart';
+
+/// Channel list rendered inside the nav drawer.
+///
+/// Lets you see every channel's unread count and jump straight to another
+/// channel without backing out of the one you are reading.
+///
+/// [currentChannelIndex] highlights the channel you are in, and is null on the
+/// channels list screen where no channel is open.
+class ChannelDrawerList extends StatelessWidget {
+  final int? currentChannelIndex;
+  final ValueChanged<Channel> onChannelSelected;
+
+  const ChannelDrawerList({
+    super.key,
+    required this.onChannelSelected,
+    this.currentChannelIndex,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    final channels = context.watch<MeshCoreConnector>().channels;
+
+    if (channels.isEmpty) {
+      return Center(
+        child: Padding(
+          padding: const EdgeInsets.all(24),
+          child: Text(
+            context.l10n.channels_title,
+            style: Theme.of(context).textTheme.bodyMedium,
+            textAlign: TextAlign.center,
+          ),
+        ),
+      );
+    }
+
+    return ListView.builder(
+      padding: EdgeInsets.zero,
+      itemCount: channels.length,
+      itemBuilder: (context, i) {
+        final channel = channels[i];
+        return _ChannelDrawerTile(
+          channel: channel,
+          selected: channel.index == currentChannelIndex,
+          onTap: () => onChannelSelected(channel),
+        );
+      },
+    );
+  }
+}
+
+class _ChannelDrawerTile extends StatelessWidget {
+  final Channel channel;
+  final bool selected;
+  final VoidCallback onTap;
+
+  const _ChannelDrawerTile({
+    required this.channel,
+    required this.selected,
+    required this.onTap,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    // Selector keeps the rebuild to this tile when its unread count moves.
+    final unread = context.select<MeshCoreConnector, int>(
+      (c) => c.getUnreadCountForChannelIndex(channel.index),
+    );
+    final theme = Theme.of(context);
+
+    return ListTile(
+      dense: true,
+      selected: selected,
+      selectedTileColor: theme.colorScheme.secondaryContainer,
+      leading: Icon(
+        channel.isPublicChannel ? Icons.public : Icons.tag,
+        size: 20,
+      ),
+      title: Text(
+        channel.name,
+        maxLines: 1,
+        overflow: TextOverflow.ellipsis,
+        style: TextStyle(
+          fontWeight: unread > 0 ? FontWeight.w700 : FontWeight.w500,
+        ),
+      ),
+      trailing: unread > 0 ? UnreadBadge(count: unread) : null,
+      onTap: onTap,
+    );
+  }
+}