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/data/models/knowledge_item.dart

@@ -52,7 +52,7 @@ class KnowledgeItem {
         id: json['id'] as int,
         noteType: json['note_type'] as String? ?? 'note',
         title: json['title'] as String? ?? '',
-        body: json['body'] as String? ?? '',
+        body: (json['snippet'] ?? json['body']) as String? ?? '',
         tags: (json['tags'] as List<dynamic>?)
                 ?.map((e) => e as String)
                 .toList() ??