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_en.arb

@@ -85,7 +85,7 @@
   "@pathImportFailed": {"placeholders": {"reason": {"type": "String"}}},
   "pathImportUnreadable": "It isn't text in UTF-8.",
   "pathsEmpty": "No paths yet",
-  "pathsEmptyHint": "A path leads through a lesson, step by step.",
+  "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.",
   "pathOptions": "Path options",
   "pathEdit": "Edit",
   "pathResetProgress": "Reset progress",
@@ -217,7 +217,7 @@
   "@studysetCountPaths": {"placeholders": {"count": {"type": "int"}}},
   "studysetCreateNote": "Create a note",
   "notesEmpty": "No notes yet",
-  "notesEmptyHint": "Notes will appear here when you add them.",
+  "notesEmptyHint": "Use \"Add material → Create a note\" to write one.",
   "noteUntitled": "Untitled note",
   "noteEdited": "Edited {when}",
   "@noteEdited": {"placeholders": {"when": {"type": "String"}}},
@@ -247,7 +247,7 @@
   "noteAddTags": "Add tags",
   "noteEditTags": "Edit tags",
   "studysetFlashcardsEmpty": "No flashcards yet",
-  "studysetFlashcardsEmptyHint": "Add some to start studying.",
+  "studysetFlashcardsEmptyHint": "Use \"Add material → Create flashcards\" to write some, or \"⋮ → Import content\" to bring in an Anki deck or a list.",
   "studysetAddMaterial": "Add material",
   "studysetCreateFlashcards": "Create flashcards",
   "studysetStudy": "Study",