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.dart

@@ -517,7 +517,7 @@ abstract class AppLocalizations {
   /// No description provided for @pathsEmptyHint.
   ///
   /// In en, this message translates to:
-  /// **'A path leads through a lesson, step by step.'**
+  /// **'A path leads through a lesson, step by step. Use \"Add material → Create a path\" to write one, or \"⋮ → Import content\" to import one.'**
   String get pathsEmptyHint;
 
   /// No description provided for @pathOptions.
@@ -1255,7 +1255,7 @@ abstract class AppLocalizations {
   /// No description provided for @notesEmptyHint.
   ///
   /// In en, this message translates to:
-  /// **'Notes will appear here when you add them.'**
+  /// **'Use \"Add material → Create a note\" to write one.'**
   String get notesEmptyHint;
 
   /// No description provided for @noteUntitled.
@@ -1411,7 +1411,7 @@ abstract class AppLocalizations {
   /// No description provided for @studysetFlashcardsEmptyHint.
   ///
   /// In en, this message translates to:
-  /// **'Add some to start studying.'**
+  /// **'Use \"Add material → Create flashcards\" to write some, or \"⋮ → Import content\" to bring in an Anki deck or a list.'**
   String get studysetFlashcardsEmptyHint;
 
   /// No description provided for @studysetAddMaterial.