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/briefing/briefing_screen.dart

@@ -266,7 +266,8 @@ class _BriefingScreenState extends ConsumerState<BriefingScreen>
           ),
         ),
         data: (conv) {
-          return Column(
+          final isWide = MediaQuery.of(context).size.width >= 600;
+          Widget body = Column(
             children: [
               Expanded(
                 child: RefreshIndicator(
@@ -392,6 +393,15 @@ class _BriefingScreenState extends ConsumerState<BriefingScreen>
               ),
             ],
           );
+          if (isWide) {
+            body = Center(
+              child: ConstrainedBox(
+                constraints: const BoxConstraints(maxWidth: 700),
+                child: body,
+              ),
+            );
+          }
+          return body;
         },
       ),
     );