feat(app): every empty studyset tab says how to add some, as Files did

Paths, flashcards and notes now show what Files showed when empty: an
icon for what the tab holds, "No … yet", and the menus that add some
("Add material → Create a path", "⋮ → Import content", …), through one
shared widget, EmptyListMessage. The icons are those the app uses for
each already: the route, the study screen's card stack, the note pen,
the folder.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/l10n/app_localizations_en.dart

@@ -295,7 +295,8 @@ class AppLocalizationsEn extends AppLocalizations {
   String get pathsEmpty => 'No paths yet';
 
   @override
-  String get pathsEmptyHint => 'A path leads through a lesson, step by step.';
+  String get pathsEmptyHint =>
+      'A path leads through a lesson, step by step. Use \"Add material → Create a path\" to write one, or \"⋮ → Import content\" to import one.';
 
   @override
   String get pathOptions => 'Path options';
@@ -713,7 +714,8 @@ class AppLocalizationsEn extends AppLocalizations {
   String get notesEmpty => 'No notes yet';
 
   @override
-  String get notesEmptyHint => 'Notes will appear here when you add them.';
+  String get notesEmptyHint =>
+      'Use \"Add material → Create a note\" to write one.';
 
   @override
   String get noteUntitled => 'Untitled note';
@@ -819,7 +821,8 @@ class AppLocalizationsEn extends AppLocalizations {
   String get studysetFlashcardsEmpty => 'No flashcards yet';
 
   @override
-  String get studysetFlashcardsEmptyHint => 'Add some to start studying.';
+  String get studysetFlashcardsEmptyHint =>
+      'Use \"Add material → Create flashcards\" to write some, or \"⋮ → Import content\" to bring in an Anki deck or a list.';
 
   @override
   String get studysetAddMaterial => 'Add material';