feat: upgrade Riverpod 3, go_router 17, google_fonts 8, package_info_plus 9

Migrate StateNotifierProvider/StateNotifier → NotifierProvider/Notifier,
StateProvider → NotifierProvider with simple notifier, FamilyAsyncNotifier
removed in Riverpod 3 (replaced with constructor-arg pattern), and rename
.valueOrNull → .value throughout all providers and screens.

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

lib/providers/briefing_provider.dart

@@ -5,7 +5,13 @@ import '../data/models/message.dart';
 import 'api_client_provider.dart';
 
 /// Drives the loading indicator in BriefingScreen's reply area.
-final isBriefingStreamingProvider = StateProvider<bool>((ref) => false);
+final isBriefingStreamingProvider =
+    NotifierProvider<_BoolNotifier, bool>(_BoolNotifier.new);
+
+class _BoolNotifier extends Notifier<bool> {
+  @override
+  bool build() => false;
+}
 
 final briefingProvider =
     AsyncNotifierProvider<BriefingNotifier, BriefingConversation>(
@@ -32,7 +38,7 @@ class BriefingNotifier extends AsyncNotifier<BriefingConversation> {
   ///   3. SSE stream (best-effort)
   ///   4. Poll until complete
   Future<void> sendReply(String content) async {
-    final conv = state.valueOrNull;
+    final conv = state.value;
     if (conv == null) return;
     final convId = conv.id;
     final chatApi = ref.read(chatApiProvider);
@@ -65,7 +71,7 @@ class BriefingNotifier extends AsyncNotifier<BriefingConversation> {
     try {
       await for (final chunk in chatApi.streamGeneration(convId)) {
         streamedContent = true;
-        final current = state.valueOrNull;
+        final current = state.value;
         if (current == null) break;
         final msgs = current.messages;
         if (msgs.isEmpty) continue;
@@ -88,7 +94,7 @@ class BriefingNotifier extends AsyncNotifier<BriefingConversation> {
         final hasContent = fresh.any(
           (m) => m.role == MessageRole.assistant && m.content.isNotEmpty,
         );
-        final current = state.valueOrNull;
+        final current = state.value;
         if (current != null && (!streamedContent || done || hasContent)) {
           state = AsyncData(current.copyWith(messages: fresh));
         }
@@ -96,7 +102,7 @@ class BriefingNotifier extends AsyncNotifier<BriefingConversation> {
       }
     } catch (_) {
       // Clear the generating placeholder so UI doesn't spin forever.
-      final current = state.valueOrNull;
+      final current = state.value;
       if (current != null) {
         final msgs = current.messages;
         if (msgs.isNotEmpty && msgs.last.status == 'generating') {