feat(todo,app): files attached to tasks and comments

Halkyon's ADR 0043, D1, D3, D5 and D8: any file up to 100 MB, attached to
a task's description or to a comment, sealed as everything of Todo's.

Server: the attachment table, synced and sealed as the others, with the
sealed file's hash readable for the store; todoFiles hands halkyon_base
its rows, readable by their space's members, and takes sealed files
alone; the files no attachment names go daily. Its tests hold downloads
to the row's space, refuse a readable file and prune what no live row
names. Migration attachments: CREATE TABLE "attachment" with its indexes
and the foreign key to its space, cascading as every synced table's.

App: AttachmentRepository seals each file for its row, keeps it sealed
and opened in the person's folder, uploads it after each sync and fetches
it as it shows, both held back by Without files on mobile data but for a
file tapped. Under the description and a comment's text, a row of them:
pictures as thumbnails opening full-size, other files as chips opening in
the system's app, else saved; the paperclip beside the description and in the comment's
composer adds them, the composer holding them until it is sent. A task deleted takes its files
and its comments' along, a comment its own.

halkyon_app opens a file in the system's app (FileOpener): on Android
through its plugin and a FileProvider of its own class, sharing the
cache's halkyon_opened/ folder alone; on the desktop as a file: URI.

Whats-New: Attach files to a task's description or a comment with the paperclip: pictures show as thumbnails, other files open in their app.
Whats-New-De: Hänge mit der Büroklammer Dateien an die Beschreibung einer Aufgabe oder an einen Kommentar: Bilder erscheinen als Vorschau, andere Dateien öffnen sich in ihrer App.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_app/lib/halkyon_app.dart

@@ -100,6 +100,7 @@ export 'src/auth/widget/start_screen.dart';
 export 'src/auth/widget/wait_screen.dart';
 export 'src/base/base_client.dart';
 export 'src/files/file_cache.dart';
+export 'src/files/file_opener.dart';
 export 'src/files/file_picker.dart';
 export 'src/files/file_saver.dart';
 export 'src/files/file_server_gateway.dart';

packages/halkyon_app/lib/src/files/file_opener.dart

@@ -0,0 +1,74 @@
+import 'dart:io';
+
+import 'package:crypto/crypto.dart';
+import 'package:flutter/services.dart';
+import 'package:path/path.dart' as p;
+import 'package:url_launcher/url_launcher.dart';
+
+/// Opens a file in the app the system picks for it (Halkyon's ADR 0043, D1):
+/// an attachment's PDF in the PDF viewer, its spreadsheet in an office app.
+/// A fake in tests.
+abstract interface class FileOpener() {
+  /// Opens [bytes], a file named [name] of [mimeType]; whether an app took
+  /// it (false: none could, or the platform opens none, iOS).
+  Future<bool> open(
+    Uint8List bytes, {
+    required String name,
+    required String mimeType,
+  });
+}
+
+/// [FileOpener] as the platform opens files: on Android through halkyon_app's
+/// own plugin (`HalkyonAppPlugin.java`, a content URI from its FileProvider,
+/// readable by the app it goes to alone), on the desktop as a `file:` URI.
+/// Either way it opens a copy under its name, which other apps show, in
+/// the cache's `halkyon_opened/` folder (Android's provider shares that folder
+/// alone), and leaves the files the app keeps as they are; the copies go
+/// with the cache, and each time this opens one.
+class SystemFileOpener(final Directory _cache) implements FileOpener {
+  static const _files = MethodChannel('app.halkyon/files');
+
+  @override
+  Future<bool> open(
+    Uint8List bytes, {
+    required String name,
+    required String mimeType,
+  }) async {
+    if (!(Platform.isAndroid ||
+        Platform.isLinux ||
+        Platform.isMacOS ||
+        Platform.isWindows)) {
+      return false;
+    }
+    final folder = Directory(p.join(_cache.path, 'halkyon_opened'));
+    if (folder.existsSync()) {
+      // Those opened before are done with: an app holding one open keeps
+      // reading it where the system allows.
+      await folder.delete(recursive: true);
+    }
+    // A folder by its content, so two files of one name don't meet.
+    final file = File(
+      p.join(
+        folder.path,
+        sha256.convert(bytes).toString().substring(0, 16),
+        _safeName(name),
+      ),
+    );
+    await file.parent.create(recursive: true);
+    await file.writeAsBytes(bytes, flush: true);
+    if (Platform.isAndroid) {
+      return await _files.invokeMethod<bool>('open', {
+            'path': file.path,
+            'mimeType': mimeType,
+          }) ??
+          false;
+    }
+    return await launchUrl(Uri.file(file.path));
+  }
+
+  /// [name] as one file name, nothing of a path in it.
+  static String _safeName(String name) {
+    final base = p.basename(name.replaceAll(r'\', '/')).trim();
+    return base.isEmpty || base == '.' || base == '..' ? 'file' : base;
+  }
+}

packages/halkyon_app/lib/src/testing/fake_file_opener.dart

@@ -0,0 +1,22 @@
+import 'dart:typed_data';
+
+import 'package:halkyon_app/src/files/file_opener.dart';
+
+/// A [FileOpener] that notes what it was asked to open.
+class FakeFileOpener() implements FileOpener {
+  /// What was opened, in order.
+  final opened = <({String name, String mimeType, Uint8List bytes})>[];
+
+  /// Whether an app takes what is opened.
+  bool takes = true;
+
+  @override
+  Future<bool> open(
+    Uint8List bytes, {
+    required String name,
+    required String mimeType,
+  }) async {
+    opened.add((name: name, mimeType: mimeType, bytes: bytes));
+    return takes;
+  }
+}

packages/halkyon_app/lib/testing.dart

@@ -12,6 +12,7 @@ export 'src/testing/fake_account_crypto.dart';
 export 'src/testing/fake_assistant_gateway.dart';
 export 'src/testing/fake_auth_gateway.dart';
 export 'src/testing/fake_content_cipher.dart';
+export 'src/testing/fake_file_opener.dart';
 export 'src/testing/fake_file_picker.dart';
 export 'src/testing/fake_file_server_gateway.dart';
 export 'src/testing/fake_instance_checker.dart';