feat(app): the card composer's state: many cards, saved on leaving

A bloc holding a studyset's cards as drafts, new ones on top and the
stored ones below. A card is saved when the person leaves it, once it
changed and has everything its type asks for, only its type's fields
stored; a failed save waits for the next leaving. A new card takes the
type of the card before it and what is pinned of it (front, answer,
tags); duplicating, clearing and deleting; a card devices changed
differently first needs a version chosen for every field in conflict.

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/test/support/fake_card_gateway.dart

@@ -19,6 +19,9 @@ class FakeCardGateway() implements CardGateway {
   /// The deletion steps, in order: `blank <card id>`, `delete <card id>`.
   final deletions = <String>[];
 
+  /// When set, storing a card or a revision fails with it.
+  Exception? failWrites;
+
   /// The payload each revision holds now, by revision id.
   Map<String, ByteData> get payloads => {
     for (final revision in _revisions.values) revision.id: revision.payload,
@@ -59,6 +62,9 @@ class FakeCardGateway() implements CardGateway {
     required String position,
     required ByteData payload,
   }) async {
+    if (failWrites case final error?) {
+      throw error;
+    }
     final id = _newId('card');
     _cards[id] = (studysetId: studysetId, position: position);
     await insertRevision(cardId: id, parents: const {}, payload: payload);
@@ -71,6 +77,9 @@ class FakeCardGateway() implements CardGateway {
     required Set<String> parents,
     required ByteData payload,
   }) async {
+    if (failWrites case final error?) {
+      throw error;
+    }
     final revision = (
       id: _newId('rev'),
       cardId: cardId,