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/screens/chat/chat_screen.dart

@@ -70,7 +70,7 @@ class _ChatScreenState extends ConsumerState<ChatScreen> {
 
     final convTitle = ref
         .watch(conversationsProvider)
-        .valueOrNull
+        .value
         ?.where((c) => c.id == widget.conversationId)
         .firstOrNull
         ?.title;