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;

lib/screens/library/library_screen.dart

@@ -228,8 +228,8 @@ class _LibraryScreenState extends ConsumerState<LibraryScreen> {
     AsyncValue<List<Note>> notesAsync,
     AsyncValue<List<Task>> tasksAsync,
   ) {
-    final notes = notesAsync.valueOrNull ?? [];
-    final tasks = tasksAsync.valueOrNull ?? [];
+    final notes = notesAsync.value ?? [];
+    final tasks = tasksAsync.value ?? [];
 
     // Merge and sort by updatedAt desc
     final items = <(DateTime, Widget)>[];

lib/screens/notes/note_detail_screen.dart

@@ -16,7 +16,7 @@ class NoteDetailScreen extends ConsumerWidget {
   @override
   Widget build(BuildContext context, WidgetRef ref) {
     final noteAsync = ref.watch(noteDetailProvider(noteId));
-    final allNotes = ref.watch(notesProvider).valueOrNull ?? [];
+    final allNotes = ref.watch(notesProvider).value ?? [];
 
     void navigateByTitle(String title) {
       final matches = allNotes.where(