feat(app): choose, manage and filter by tags

The card editor offers its tags: a picker that searches by the start of
a name, creates a tag (name and colour) and chooses it, and saves the
choice with the card. A sheet lists every tag with its count, to add,
rename (merging into a tag of that name), recolour and delete them, with
an undo. The card list shows each card's tags and narrows to one tag.

The editor closed twice once tags changed after saving; it now closes
on becoming done only.

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/src/app/app_router.dart

@@ -105,23 +105,29 @@ GoRouter buildRouter({String initialLocation = RoutePaths.home}) => GoRouter(
       builder: (context, state) => BlocProvider(
         create: (context) => CardEditBloc(
           PersonScope.of(context).cards,
+          PersonScope.of(context).tags,
           state.pathParameters['studysetId']!,
           null,
-        ),
+        )..add(const CardEditEventStarted()),
         child: const CardScreen(card: null),
       ),
     ),
     GoRoute(
       path: '/studysets/:studysetId/cards/:cardId',
       builder: (context, state) {
-        final cards = PersonScope.of(context).cards;
+        final person = PersonScope.of(context);
         final studysetId = state.pathParameters['studysetId']!;
         return FoundById<Card>(
           id: state.pathParameters['cardId']!,
-          items: () => cards.watch(studysetId),
+          items: () => person.cards.watch(studysetId),
           idOf: (card) => card.id,
           builder: (card, _) => BlocProvider(
-            create: (context) => CardEditBloc(cards, studysetId, card),
+            create: (context) => CardEditBloc(
+              person.cards,
+              person.tags,
+              studysetId,
+              card,
+            )..add(const CardEditEventStarted()),
             child: CardScreen(card: card),
           ),
         );