fix(#390): hide dead top-level back-arrow on pinned desktop

Top-level tabs (Contacts/Channels/Map) no longer auto-imply an AppBar
leading. They build via appBarBuilder(pinned): no leading when the nav
panel is pinned (desktop), the drawer hamburger when transient (mobile).
Detail screens are untouched and keep their working back button.

Removes the unused hideBackButton constructor param (declared, passed
true at quick-switch call sites, never read) and all its call sites.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/screens/channel_chat_screen.dart

@@ -309,9 +309,7 @@ class _ChannelChatScreenState extends State<ChannelChatScreen> {
     navigator.pop();
     navigator.pushReplacement(
       buildQuickSwitchRoute(
-        index == 0
-            ? const ContactsScreen(hideBackButton: true)
-            : const MapScreen(hideBackButton: true),
+        index == 0 ? const ContactsScreen() : const MapScreen(),
       ),
     );
   }

lib/screens/channels_screen.dart

@@ -35,9 +35,7 @@ import 'map_screen.dart';
 import 'settings_screen.dart';
 
 class ChannelsScreen extends StatefulWidget {
-  final bool hideBackButton;
-
-  const ChannelsScreen({super.key, this.hideBackButton = false});
+  const ChannelsScreen({super.key});
 
   @override
   State<ChannelsScreen> createState() => _ChannelsScreenState();
@@ -110,7 +108,11 @@ class _ChannelsScreenState extends State<ChannelsScreen>
         context,
         MaterialPageRoute(builder: (context) => const SettingsScreen()),
       ),
-      appBar: AppBar(
+      appBarBuilder: (context, pinned) => AppBar(
+        // Top-level tab: no auto back-arrow. Hamburger when the panel is not
+        // pinned; nothing when it is (panel is docked), so no dead arrow (#390).
+        automaticallyImplyLeading: false,
+        leading: pinned ? null : AppShell.drawerMenuButton(),
         title: AppBarTitle(context.l10n.channels_title),
         centerTitle: true,
         bottom: const SyncProgressAppBarBottom(),
@@ -570,13 +572,13 @@ class _ChannelsScreenState extends State<ChannelsScreen>
       case 0:
         Navigator.pushReplacement(
           context,
-          buildQuickSwitchRoute(const ContactsScreen(hideBackButton: true)),
+          buildQuickSwitchRoute(const ContactsScreen()),
         );
         break;
       case 2:
         Navigator.pushReplacement(
           context,
-          buildQuickSwitchRoute(const MapScreen(hideBackButton: true)),
+          buildQuickSwitchRoute(const MapScreen()),
         );
         break;
     }

lib/screens/contacts_screen.dart

@@ -48,9 +48,7 @@ enum RoomLoginDestination { chat, management }
 enum ContactOperationType { import, export, zeroHopShare }
 
 class ContactsScreen extends StatefulWidget {
-  final bool hideBackButton;
-
-  const ContactsScreen({super.key, this.hideBackButton = false});
+  const ContactsScreen({super.key});
 
   @override
   State<ContactsScreen> createState() => _ContactsScreenState();
@@ -329,7 +327,11 @@ class _ContactsScreenState extends State<ContactsScreen>
         context,
         MaterialPageRoute(builder: (context) => const SettingsScreen()),
       ),
-      appBar: AppBar(
+      appBarBuilder: (context, pinned) => AppBar(
+        // Top-level tab: no auto back-arrow. Hamburger when the panel is not
+        // pinned; nothing when it is (panel is docked), so no dead arrow (#390).
+        automaticallyImplyLeading: false,
+        leading: pinned ? null : AppShell.drawerMenuButton(),
         title: AppBarTitle(context.l10n.contacts_title),
         bottom: const SyncProgressAppBarBottom(),
         actions: [
@@ -949,13 +951,13 @@ class _ContactsScreenState extends State<ContactsScreen>
       case 1:
         Navigator.pushReplacement(
           context,
-          buildQuickSwitchRoute(const ChannelsScreen(hideBackButton: true)),
+          buildQuickSwitchRoute(const ChannelsScreen()),
         );
         break;
       case 2:
         Navigator.pushReplacement(
           context,
-          buildQuickSwitchRoute(const MapScreen(hideBackButton: true)),
+          buildQuickSwitchRoute(const MapScreen()),
         );
         break;
     }

lib/screens/line_of_sight_map_screen.dart

@@ -1362,13 +1362,13 @@ class _LineOfSightMapScreenState extends State<LineOfSightMapScreen> {
       case 0:
         Navigator.pushReplacement(
           context,
-          buildQuickSwitchRoute(const ContactsScreen(hideBackButton: true)),
+          buildQuickSwitchRoute(const ContactsScreen()),
         );
         break;
       case 1:
         Navigator.pushReplacement(
           context,
-          buildQuickSwitchRoute(const ChannelsScreen(hideBackButton: true)),
+          buildQuickSwitchRoute(const ChannelsScreen()),
         );
         break;
     }

lib/screens/map_screen.dart

@@ -42,7 +42,6 @@ class MapScreen extends StatefulWidget {
   final String? highlightLabel;
   final String? highlightMarkerKey;
   final double highlightZoom;
-  final bool hideBackButton;
 
   const MapScreen({
     super.key,
@@ -50,7 +49,6 @@ class MapScreen extends StatefulWidget {
     this.highlightLabel,
     this.highlightMarkerKey,
     this.highlightZoom = 15.0,
-    this.hideBackButton = false,
   });
 
   @override
@@ -421,7 +419,12 @@ class _MapScreenState extends State<MapScreen> {
             context,
             MaterialPageRoute(builder: (context) => const SettingsScreen()),
           ),
-          appBar: AppBar(
+          appBarBuilder: (context, pinned) => AppBar(
+            // Top-level tab: no auto back-arrow. Hamburger when the panel is
+            // not pinned; nothing when pinned (panel docked), so no dead
+            // arrow (#390).
+            automaticallyImplyLeading: false,
+            leading: pinned ? null : AppShell.drawerMenuButton(),
             title: AppBarTitle(context.l10n.map_title),
             centerTitle: true,
             bottom: const SyncProgressAppBarBottom(),
@@ -1636,13 +1639,13 @@ class _MapScreenState extends State<MapScreen> {
       case 0:
         Navigator.pushReplacement(
           context,
-          buildQuickSwitchRoute(const ContactsScreen(hideBackButton: true)),
+          buildQuickSwitchRoute(const ContactsScreen()),
         );
         break;
       case 1:
         Navigator.pushReplacement(
           context,
-          buildQuickSwitchRoute(const ChannelsScreen(hideBackButton: true)),
+          buildQuickSwitchRoute(const ChannelsScreen()),
         );
         break;
     }

lib/widgets/app_shell.dart

@@ -89,6 +89,20 @@ class AppShell extends StatefulWidget {
     return AppShellBackAction.background;
   }
 
+  /// Hamburger for a top-level tab's app-bar leading, used when the nav panel
+  /// is NOT pinned (transient drawer). Top-level tabs must set
+  /// `automaticallyImplyLeading: false` and use this instead, so the pinned
+  /// desktop layout shows no leading at all rather than a dead back-arrow that
+  /// only backgrounds (a no-op on desktop): the #390 eyesore. A [Builder] so
+  /// `openDrawer` resolves a context beneath the Scaffold.
+  static Widget drawerMenuButton() => Builder(
+    builder: (context) => IconButton(
+      icon: const Icon(Icons.menu),
+      tooltip: MaterialLocalizations.of(context).openAppDrawerTooltip,
+      onPressed: () => Scaffold.of(context).openDrawer(),
+    ),
+  );
+
   @override
   State<AppShell> createState() => _AppShellState();
 }