feat: silent background polling for briefing screen

Timer.periodic every 60s calls silentRefresh() on BriefingNotifier.
silentRefresh() patches AsyncData directly — never triggers AsyncLoading —
so existing content stays on screen while the fetch is in flight.
WidgetsBindingObserver pauses polling when app is backgrounded.
Polling is also skipped while a reply is actively streaming.

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

lib/providers/briefing_provider.dart

@@ -23,6 +23,24 @@ class BriefingNotifier extends AsyncNotifier<BriefingConversation> {
     return ref.read(briefingApiProvider).getToday();
   }
 
+  /// Silently fetch the latest briefing and patch state without triggering
+  /// AsyncLoading — existing content stays visible while the fetch is in flight.
+  Future<void> silentRefresh() async {
+    final current = state.value;
+    if (current == null) return;
+    try {
+      final fresh = await ref.read(briefingApiProvider).getToday();
+      final curLast = current.messages.isNotEmpty ? current.messages.last : null;
+      final newLast = fresh.messages.isNotEmpty ? fresh.messages.last : null;
+      if (fresh.messages.length != current.messages.length ||
+          newLast?.content != curLast?.content) {
+        state = AsyncData(fresh);
+      }
+    } catch (_) {
+      // Network hiccup — silently ignore, keep existing content
+    }
+  }
+
   /// Trigger a briefing slot (e.g. "compilation") then reload.
   Future<void> refresh(String slot) async {
     await ref.read(briefingApiProvider).triggerSlot(slot);