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>

apps/todo/halkyon_todo_flutter/lib/l10n/app_localizations.dart

@@ -2739,6 +2739,78 @@ abstract class AppLocalizations {
   /// In en, this message translates to:
   /// **'Weather data by Open-Meteo.com'**
   String get weatherBy;
+
+  /// The paperclip button adding files to a task's description or a comment.
+  ///
+  /// In en, this message translates to:
+  /// **'Attach files'**
+  String get attachmentAdd;
+
+  /// The file type offered in the dialog picking files to attach.
+  ///
+  /// In en, this message translates to:
+  /// **'Any file'**
+  String get attachmentAnyFile;
+
+  /// Said when a file picked to attach is too large.
+  ///
+  /// In en, this message translates to:
+  /// **'{name} is larger than 100 MB.'**
+  String attachmentTooLarge(String name);
+
+  /// Said when a file couldn't be attached (no keys on this device, or it failed).
+  ///
+  /// In en, this message translates to:
+  /// **'{name} wasn\'t attached.'**
+  String attachmentNotAdded(String name);
+
+  /// The tooltip of an attachment not on this device yet, fetched when tapped.
+  ///
+  /// In en, this message translates to:
+  /// **'Download'**
+  String get attachmentDownload;
+
+  /// Said when an attachment couldn't be downloaded.
+  ///
+  /// In en, this message translates to:
+  /// **'This file can\'t be fetched right now. Are you online?'**
+  String get attachmentUnavailable;
+
+  /// The menu entry saving an attachment where the person chooses.
+  ///
+  /// In en, this message translates to:
+  /// **'Save a copy'**
+  String get attachmentSave;
+
+  /// The menu entry removing an attachment, and the button removing a file picked for a comment before it is sent.
+  ///
+  /// In en, this message translates to:
+  /// **'Remove'**
+  String get attachmentRemove;
+
+  /// The title of the question before an attachment is removed.
+  ///
+  /// In en, this message translates to:
+  /// **'Remove this file?'**
+  String get attachmentRemoveTitle;
+
+  /// The tooltip of an attachment's menu.
+  ///
+  /// In en, this message translates to:
+  /// **'File options'**
+  String get attachmentMenu;
+
+  /// An attachment's size in kilobytes.
+  ///
+  /// In en, this message translates to:
+  /// **'{size} KB'**
+  String attachmentSizeKb(int size);
+
+  /// An attachment's size in megabytes, with one decimal.
+  ///
+  /// In en, this message translates to:
+  /// **'{size} MB'**
+  String attachmentSizeMb(String size);
 }
 
 class _AppLocalizationsDelegate