feat(app): notes in an order of one's own, with Reorder beside the tabs

With two notes or more, Reorder beside the tabs gives each note a grip
in place of its options, as paths have, until Done; e.g. to follow a
reading order. A note's position was stored already; the repository
now moves it (only that column changes, and it syncs).

The notes' bloc sits with the studyset's other lists now, above the
tabs, so the button knows how many there are. The position helpers the
paths kept to themselves are shared (core/sync/fractional_position.dart),
and so is the grip (ReorderGrip).

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

@@ -1023,6 +1023,9 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get studysetReorderPaths => 'Reorder paths';
 
+  @override
+  String get studysetReorderNotes => 'Reorder notes';
+
   @override
   String get studysetReorderDone => 'Done';