refactor(app,learn): people's files on the device shared in halkyon_app

ADR 0043's recommendation, for Todo's attachments: what Learn had for its
files on the device moves into halkyon_app, unchanged but for its names,
and Learn uses it from there.

- FileCache, DirectoryFileCache (Learn's AssetFileCache): the files by
  their SHA-256 in a folder of the person's, and which wait to be
  uploaded.
- FileServerGateway, ServerpodFileServerGateway (Learn's
  AssetServerGateway): halkyon_base's files endpoint, taking the module's
  caller rather than Learn's client; FileLimitReached, FileRoom.
- Their fakes in halkyon_app's testing: MemoryFileCache,
  FakeFileServerGateway, sha256Of.
- SealedFile: the format every app seals a file in (HLKF, version 1),
  sealing with halkyon_app's ContentCipher; Learn's Privacy seals and
  opens its files with it, for the same context as before, so every file
  sealed so far opens as it did.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/learn/halkyon_learn_flutter/test/feature/anki/widget/anki_import_sheet_test.dart

@@ -15,7 +15,6 @@ import 'package:halkyon_learn_flutter/src/feature/studyset/data/studyset_reposit
 import 'package:halkyon_learn_flutter/src/feature/studyset/model/studyset_content.dart';
 import 'package:material_ui/material_ui.dart';
 
-import '../../../support/fake_asset_server_gateway.dart';
 import '../../../support/fake_card_gateway.dart';
 import '../../../support/fake_note_gateway.dart';
 import '../../../support/fake_studyset_gateway.dart';
@@ -26,13 +25,13 @@ import '../data/anki_package_test.dart' show fixture;
 void main() {
   late FakeCardGateway cardGateway;
   late FakeFilePicker files;
-  late FakeAssetServerGateway server;
+  late FakeFileServerGateway server;
   late FakeNoteGateway noteGateway;
 
   setUp(() {
     cardGateway = FakeCardGateway();
     files = FakeFilePicker();
-    server = FakeAssetServerGateway();
+    server = FakeFileServerGateway();
     noteGateway = FakeNoteGateway();
   });