Add ISO dates to search so we can query by them

lib/meilisearch.dart

@@ -350,6 +350,7 @@ Future<Note> createNote(String content) async {
   final document = {
     'id': generateRandomString(32),
     'date': DateTime.now().toUtc().millisecondsSinceEpoch,
+    'dateISO': DateTime.now().toUtc().toIso8601String(),
     'content': content,
     'topLetter': mostFrequentLetter,
     'topLetterFrequency': mostFrequentLetterCount,
@@ -433,6 +434,10 @@ Future<void> updateNote(Note note) async {
     'id': note.id,
     'content': trimmedContent,
     'date': note.epochTime,
+    'dateISO':
+        DateTime.fromMillisecondsSinceEpoch(
+          note.epochTime,
+        ).toUtc().toIso8601String(),
     'topLetter': mostFrequentLetter,
     'topLetterFrequency': mostFrequentLetterRatio,
   };