fix(#289): make the nav panel pinnable inside a channel too

The channel chat screen had a plain Drawer bolted onto it, so the panel could
only be pinned open on the four primary views. Pinning it while reading a
channel is the wide-screen case that was actually asked for: keeping the
channel list and its unread counts visible without pulling it out.

- AppShell now serves pushed detail screens as well: selectedIndex and
  onDestinationSelected are optional, and the bottom bar is omitted when they
  are absent (a pushed chat screen has no bottom bar).
- New appBarBuilder(context, pinned) lets a screen vary its app bar with the
  dock state. ChannelChatScreen uses it to drop its hamburger when the panel
  is already pinned open, since there is nothing left to summon.
- ChannelChatScreen builds on AppShell instead of a bare Scaffold, so it gets
  the same transient/pinned layout as everything else.

The four primary views are untouched; the new parameters are additive.

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/app_shell.dart';
 import '../widgets/channel_drawer_list.dart';
 import '../widgets/mention_autocomplete.dart';
 import '../helpers/emoji_shortcodes.dart';
@@ -294,21 +295,22 @@ class _ChannelChatScreenState extends State<ChannelChatScreen> {
 
   @override
   Widget build(BuildContext context) {
-    return Scaffold(
-      drawer: Drawer(
-        width: 300,
-        child: SafeArea(
-          child: ChannelDrawerList(
+    return AppShell(
+      // No bottom bar: this is a pushed detail screen. The panel is still
+      // pinnable here, which is the wide-screen point - keeping the channel
+      // list and its unread counts visible while reading a channel.
+      drawerContent: 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(
+      appBarBuilder: (context, pinned) => AppBar(
+        // Pinned: the panel is already docked, so no hamburger is needed.
+        // Unpinned: this is a pushed route, so Scaffold would put a back arrow
+        // in this slot; the explicit hamburger replaces it. System back still
+        // pops to the channel list.
+        leading: pinned
+            ? null
+            : Builder(
                 builder: (context) => IconButton(
                   icon: const Icon(Icons.menu),
                   tooltip: context.l10n.channels_title,

lib/widgets/app_shell.dart

@@ -15,12 +15,21 @@ class AppShell extends StatelessWidget {
   static const double wideBreakpoint = 720;
   static const double _drawerWidth = 300;
 
-  final int selectedIndex;
-  final ValueChanged<int> onDestinationSelected;
+  /// Bottom bar tab. Null on pushed detail screens (a channel chat), which
+  /// carry the nav panel but no bottom bar.
+  final int? selectedIndex;
+  final ValueChanged<int>? onDestinationSelected;
   final int contactsUnreadCount;
   final int channelsUnreadCount;
 
   final PreferredSizeWidget? appBar;
+
+  /// App bar that needs to know whether the panel is docked, so a pushed
+  /// screen can drop its hamburger when the panel is already pinned open.
+  /// Takes precedence over [appBar].
+  final PreferredSizeWidget Function(BuildContext context, bool pinned)?
+  appBarBuilder;
+
   final Widget body;
   final Widget? floatingActionButton;
 
@@ -29,10 +38,11 @@ class AppShell extends StatelessWidget {
 
   const AppShell({
     super.key,
-    required this.selectedIndex,
-    required this.onDestinationSelected,
     required this.body,
+    this.selectedIndex,
+    this.onDestinationSelected,
     this.appBar,
+    this.appBarBuilder,
     this.floatingActionButton,
     this.drawerContent,
     this.contactsUnreadCount = 0,
@@ -54,22 +64,30 @@ class AppShell extends StatelessWidget {
     );
   }
 
-  Widget _bottomBar() {
+  /// Null on detail screens, which show the panel but no bottom bar.
+  Widget? _bottomBar() {
+    final index = selectedIndex;
+    final onSelected = onDestinationSelected;
+    if (index == null || onSelected == null) return null;
     return SafeArea(
       top: false,
       child: QuickSwitchBar(
-        selectedIndex: selectedIndex,
-        onDestinationSelected: onDestinationSelected,
+        selectedIndex: index,
+        onDestinationSelected: onSelected,
         contactsUnreadCount: contactsUnreadCount,
         channelsUnreadCount: channelsUnreadCount,
       ),
     );
   }
 
+  PreferredSizeWidget? _appBar(BuildContext context, bool pinned) {
+    return appBarBuilder?.call(context, pinned) ?? appBar;
+  }
+
   /// Wide + pinned: the panel is laid out beside the body, not overlaid.
   Widget _buildPinned(BuildContext context) {
     return Scaffold(
-      appBar: appBar,
+      appBar: _appBar(context, true),
       body: SafeArea(
         top: false,
         child: Row(
@@ -93,7 +111,7 @@ class AppShell extends StatelessWidget {
   /// the hamburger into the app bar automatically.
   Widget _buildTransient(BuildContext context, bool isWide) {
     return Scaffold(
-      appBar: appBar,
+      appBar: _appBar(context, false),
       drawer: Drawer(
         width: _drawerWidth,
         child: _NavPanel(isWide: isWide, content: drawerContent),