feat(app): a card browser to search, filter, sort and change many cards at once

Browse beside the flashcards' tab opens the studyset's cards over the
tabs (with everything inside it when it has sub-studysets), laid out as
the older design had it:
- a grid of tiles (each card's two sides) or one card at a time beside
  the list (on a phone above a strip of them, swiped through), as last
  left for the studyset on this device;
- a search over front, back, reading, meanings, hints, explanation and
  tag names, katakana found as hiragana and kana by romaji;
- sorting by own order, newest, oldest, A–Z (words by reading), due
  first; a filter sheet by kind, tag and state (new, learning, due,
  buried, reported);
- choosing cards (a toggle; shift-click or long-press for a range,
  select all) to move into another or a new studyset, add or remove
  tags, start afresh, or delete with Undo; the actions in the row on
  wide windows, in a bar at the foot below.

Moving cards between studysets is new in the card repository and its
gateway (studysetId and position written, named columns).

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/src/app/app_router.dart

@@ -8,6 +8,7 @@ import 'package:halkyon_learn_flutter/src/core/router/tab_visibility.dart';
 import 'package:halkyon_learn_flutter/src/core/widget/found_by_id.dart';
 import 'package:halkyon_learn_flutter/src/feature/auth/widget/profile_screen.dart';
 import 'package:halkyon_learn_flutter/src/feature/card/bloc/card_composer_bloc.dart';
+import 'package:halkyon_learn_flutter/src/feature/card/widget/card_browser_screen.dart';
 import 'package:halkyon_learn_flutter/src/feature/card/widget/composer/card_composer_screen.dart';
 import 'package:halkyon_learn_flutter/src/feature/dictionary/widget/dictionary_screen.dart';
 import 'package:halkyon_learn_flutter/src/feature/explore/widget/explore_screen.dart';
@@ -180,6 +181,20 @@ GoRouter buildRouter({String initialLocation = RoutePaths.home}) => GoRouter(
         );
       },
     ),
+    // Browsing a studyset's cards: searching, filtering and changing many at
+    // once.
+    GoRoute(
+      path: '/studysets/:studysetId/flashcards',
+      builder: (context, state) => _inStudyset(
+        context,
+        state.pathParameters['studysetId']!,
+        (studyset, all) => CardBrowserScreen(
+          studyset: studyset,
+          all: all,
+          subtree: state.uri.queryParameters['subtree'] == '1',
+        ),
+      ),
+    ),
     // The composer: writing a studyset's cards, opened on a new card or on
     // one of them. Before the card route: its `new` would otherwise read as
     // a card's id.