feat: tablet landscape improvements

Shell: move Projects/News/Calendar into ShellRoute so the NavigationRail
persists across all screens. Show all 6 nav destinations on tablet
instead of 3+More overflow. Phone bottom nav unchanged.

Chat: master-detail layout on tablet — conversations list (320px) with
inline chat panel. Tapping a conversation updates state instead of
pushing a new route.

Knowledge: responsive grid (2 cols portrait, 3 cols landscape) with
card layout showing icon, title, body preview, and tags. Fix snippet
data — read json['snippet'] which the API actually sends. Bump snippet
length from 120 to 200 chars. Tasks now show description as snippet.

News: responsive grid with the same breakpoints. Larger snippet
(5 lines) in grid mode via snippetMaxLines parameter.

Briefing: centered reading column (maxWidth 700) on wide screens,
matching the web UI layout.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/screens/chat/conversations_tab_screen.dart

@@ -4,30 +4,79 @@ import 'package:go_router/go_router.dart';
 
 import '../../core/constants.dart';
 import '../../providers/chat_provider.dart';
+import 'chat_screen.dart';
 
-class ConversationsTabScreen extends ConsumerWidget {
+class ConversationsTabScreen extends ConsumerStatefulWidget {
   const ConversationsTabScreen({super.key});
 
   @override
-  Widget build(BuildContext context, WidgetRef ref) {
+  ConsumerState<ConversationsTabScreen> createState() =>
+      _ConversationsTabScreenState();
+}
+
+class _ConversationsTabScreenState
+    extends ConsumerState<ConversationsTabScreen> {
+  int? _selectedConvId;
+
+  Future<void> _createConversation() async {
+    final conv =
+        await ref.read(conversationsProvider.notifier).create('');
+    if (!mounted) return;
+    final isWide = MediaQuery.of(context).size.width >= 600;
+    if (isWide) {
+      setState(() => _selectedConvId = conv.id);
+    } else {
+      context.push(Routes.chat.replaceFirst(':id', '${conv.id}'));
+    }
+  }
+
+  void _openConversation(int id) {
+    final isWide = MediaQuery.of(context).size.width >= 600;
+    if (isWide) {
+      setState(() => _selectedConvId = id);
+    } else {
+      context.push(Routes.chat.replaceFirst(':id', '$id'));
+    }
+  }
+
+  Future<void> _confirmDelete(int id, String title) async {
+    final ok = await showDialog<bool>(
+      context: context,
+      builder: (dialogContext) => AlertDialog(
+        title: const Text('Delete conversation?'),
+        content: Text('"$title" will be permanently deleted.'),
+        actions: [
+          TextButton(
+              onPressed: () => Navigator.pop(dialogContext, false),
+              child: const Text('Cancel')),
+          FilledButton(
+              onPressed: () => Navigator.pop(dialogContext, true),
+              child: const Text('Delete')),
+        ],
+      ),
+    );
+    if (ok == true) {
+      await ref.read(conversationsProvider.notifier).delete(id);
+      if (_selectedConvId == id) {
+        setState(() => _selectedConvId = null);
+      }
+    }
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final isWide = MediaQuery.of(context).size.width >= 600;
     final theme = Theme.of(context);
     final convsAsync = ref.watch(conversationsProvider);
 
-    return Scaffold(
+    final listPanel = Scaffold(
       appBar: AppBar(
         title: Text('Chat', style: theme.textTheme.titleLarge),
         actions: [
           IconButton(
             icon: const Icon(Icons.add),
             tooltip: 'New conversation',
-            onPressed: () async {
-              final conv = await ref
-                  .read(conversationsProvider.notifier)
-                  .create('');
-              if (context.mounted) {
-                context.push(Routes.chat.replaceFirst(':id', '${conv.id}'));
-              }
-            },
+            onPressed: _createConversation,
           ),
         ],
       ),
@@ -49,26 +98,20 @@ class ConversationsTabScreen extends ConsumerWidget {
                   FilledButton.icon(
                     icon: const Icon(Icons.add),
                     label: const Text('Start a conversation'),
-                    onPressed: () async {
-                      final conv = await ref
-                          .read(conversationsProvider.notifier)
-                          .create('');
-                      if (context.mounted) {
-                        context.push(
-                            Routes.chat.replaceFirst(':id', '${conv.id}'));
-                      }
-                    },
+                    onPressed: _createConversation,
                   ),
                 ],
               ),
             );
           }
           return RefreshIndicator(
-            onRefresh: () => ref.read(conversationsProvider.notifier).refresh(),
+            onRefresh: () =>
+                ref.read(conversationsProvider.notifier).refresh(),
             child: ListView.builder(
               itemCount: convs.length,
               itemBuilder: (ctx, i) {
                 final c = convs[i];
+                final selected = isWide && c.id == _selectedConvId;
                 return ListTile(
                   leading: const Icon(Icons.chat_bubble_outline),
                   title: Text(
@@ -79,13 +122,12 @@ class ConversationsTabScreen extends ConsumerWidget {
                     _relativeTime(c.updatedAt),
                     style: theme.textTheme.labelSmall,
                   ),
+                  selected: selected,
                   trailing: IconButton(
                     icon: const Icon(Icons.delete_outline),
-                    onPressed: () =>
-                        _confirmDelete(context, ref, c.id, c.title),
+                    onPressed: () => _confirmDelete(c.id, c.title),
                   ),
-                  onTap: () =>
-                      ctx.push(Routes.chat.replaceFirst(':id', '${c.id}')),
+                  onTap: () => _openConversation(c.id),
                 );
               },
             ),
@@ -93,28 +135,38 @@ class ConversationsTabScreen extends ConsumerWidget {
         },
       ),
     );
-  }
 
-  Future<void> _confirmDelete(
-      BuildContext context, WidgetRef ref, int id, String title) async {
-    final ok = await showDialog<bool>(
-      context: context,
-      builder: (dialogContext) => AlertDialog(
-        title: const Text('Delete conversation?'),
-        content: Text('"$title" will be permanently deleted.'),
-        actions: [
-          TextButton(
-              onPressed: () => Navigator.pop(dialogContext, false),
-              child: const Text('Cancel')),
-          FilledButton(
-              onPressed: () => Navigator.pop(dialogContext, true),
-              child: const Text('Delete')),
-        ],
-      ),
+    if (!isWide) return listPanel;
+
+    return Row(
+      children: [
+        SizedBox(
+          width: 320,
+          child: listPanel,
+        ),
+        const VerticalDivider(width: 1),
+        Expanded(
+          child: _selectedConvId != null
+              ? ChatScreen(
+                  key: ValueKey(_selectedConvId),
+                  conversationId: _selectedConvId!,
+                )
+              : Center(
+                  child: Column(
+                    mainAxisSize: MainAxisSize.min,
+                    children: [
+                      Icon(Icons.chat_bubble_outline,
+                          size: 48,
+                          color: theme.colorScheme.onSurfaceVariant),
+                      const SizedBox(height: 16),
+                      Text('Select a conversation',
+                          style: theme.textTheme.titleMedium),
+                    ],
+                  ),
+                ),
+        ),
+      ],
     );
-    if (ok == true) {
-      await ref.read(conversationsProvider.notifier).delete(id);
-    }
   }
 }