fix(app): a recovery phrase's and codes' files name the instance

Saved from several apps or instances, they differed by " (1)" alone. They
name the instance now, without the app's word, as an operator's apps share
one account: halkyon.eu-recovery-phrase.txt, localhost-8280-recovery-codes.txt.

Whats-New: A saved recovery phrase or recovery codes file now names its instance.
Whats-New-De: Gespeicherte Wiederherstellungsphrasen und -codes nennen jetzt ihre Instanz im Dateinamen.

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/src/password/widget/password_widgets.dart

@@ -3,6 +3,7 @@ import 'dart:io';
 
 import 'package:flutter/services.dart';
 import 'package:halkyon_app/l10n/halkyon_app_localizations.dart';
+import 'package:halkyon_app/src/app/app_frame.dart';
 import 'package:halkyon_app/src/app/app_scope.dart';
 import 'package:halkyon_app/src/password/model/account_authenticator.dart';
 import 'package:halkyon_app/src/password/model/phrase_check.dart';
@@ -328,26 +329,46 @@ class const SheetProblem(final String text, {super.key})
   }
 }
 
-/// Save for [text], kept as [fileName]. Nothing secret is copied (the
-/// reading of the fixes): no app can mark a copy as sensitive through
+/// How the file of a secret names the instance called [name] (its host, with
+/// the port if it has one), for the app called [product]: without the app's
+/// word, as an operator's apps share one account (`learn.halkyon.eu`,
+/// `todo.halkyon.eu`: `halkyon.eu`), and safe in a file's name
+/// (`localhost:8280`: `localhost-8280`).
+String instanceFileLabel(String name, {required String product}) {
+  final host = name.toLowerCase();
+  final prefix = '${product.toLowerCase()}.';
+  return (host.startsWith(prefix) ? host.substring(prefix.length) : host)
+      .replaceAll(RegExp('[^a-z0-9.-]+'), '-');
+}
+
+/// Save for [text], in a file named after the instance and [fileKind].
+/// Nothing secret is copied (the reading of the fixes): no app can mark a copy as sensitive through
 /// Flutter, so clipboard histories (Windows, KDE, Gboard, Samsung's) and
 /// cloud clipboards would keep it, and on Android an app in the background
 /// can't empty the clipboard again.
 class const KeepTextButtons({
   required final String text,
-  required final String fileName,
+
+  /// What the file holds; its name adds the instance's, whose account it
+  /// belongs to whichever app showed it (`halkyon.eu-recovery-phrase.txt`).
+  required final String fileKind,
   super.key,
 }) extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final l10n = HalkyonAppLocalizations.of(context);
+    final dependencies = AppScope.of(context);
+    final instance = instanceFileLabel(
+      dependencies.serverName,
+      product: AppFrameScope.of(context).product(context),
+    );
     return Align(
       alignment: AlignmentDirectional.centerStart,
       child: OutlinedButton.icon(
         icon: const Icon(LucideIcons.download),
         label: Text(l10n.passwordSave),
-        onPressed: () => AppScope.of(context).fileSaver.save(
-          name: fileName,
+        onPressed: () => dependencies.fileSaver.save(
+          name: '$instance-$fileKind.txt',
           bytes: utf8.encode('$text\n'),
           mimeType: 'text/plain',
         ),
@@ -426,7 +447,7 @@ class const RecoveryPhraseView({required final String phrase, super.key})
           // Saved or written down, never copied ([KeepTextButtons]).
           KeepTextButtons(
             text: phrase,
-            fileName: 'halkyon-recovery-phrase.txt',
+            fileKind: 'recovery-phrase',
           ),
         ],
       ),
@@ -683,7 +704,7 @@ class const RecoveryCodesView({required final List<String> codes, super.key})
           const SizedBox(height: 12),
           KeepTextButtons(
             text: codes.join('\n'),
-            fileName: 'halkyon-recovery-codes.txt',
+            fileKind: 'recovery-codes',
           ),
         ],
       ),