feat(studyset): an empty tab says its material is in the topics, and leads there

People testing the app looked for the material of a studyset that groups
topics in its own empty tabs, which said "No flashcards yet" and how to
add some. A tab with none of its own while the topics hold some now says
so: "The flashcards are in its topics", each topic holding some with how
much (everything inside it counted), opening it; for flashcards Browse
all of them in one list; and, quieter, how to add some here after all.
Opened for the first time, such a studyset shows the first tab its topics
fill. The tabs' counts stay the studyset's own.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/l10n/app_localizations_en.dart

@@ -1033,6 +1033,45 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get studysetAddMaterial => 'Add material';
 
+  @override
+  String get studysetPathsInTopics => 'The paths are in its topics';
+
+  @override
+  String get studysetPathsInTopicsHint =>
+      'Or write one here: \"Add material → Create a path\".';
+
+  @override
+  String get studysetFlashcardsInTopics => 'The flashcards are in its topics';
+
+  @override
+  String get studysetFlashcardsInTopicsHint =>
+      'Or add some here: \"Add material → Create flashcards\".';
+
+  @override
+  String get studysetNotesInTopics => 'The notes are in its topics';
+
+  @override
+  String get studysetNotesInTopicsHint =>
+      'Or write one here: \"Add material → Create a note\".';
+
+  @override
+  String get studysetFilesInTopics => 'The files are in its topics';
+
+  @override
+  String get studysetFilesInTopicsHint =>
+      'Or add one here: \"Add material → Upload a PDF\".';
+
+  @override
+  String studysetBrowseAllFlashcards(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: 'Browse all $count',
+      one: 'Browse it',
+    );
+    return '$_temp0';
+  }
+
   @override
   String get studysetCreateFlashcards => 'Create flashcards';