feat(instances): choose the instance, and keep accounts on several

A new install asks first which instance to keep the person's studysets
on, as Mastodon's apps do: an address, checked to answer as an instance
of Halkyon Learn, or the build's own (learn.halkyon.eu) in one tap.
Nothing is made anywhere before that, not even an anonymous account. An
install from before goes on with the build's instance without asking.

As Moshidon does, the device keeps an account on each instance it was
used with, each signed in with its own folder and session, one in use at
a time. Settings › Switch account lists them: another one switches at
once, anonymous accounts included; Add account chooses another instance;
removing one not in use deletes what the device holds of it. Signing out
ends only the account in use.

The build's own instance keeps its data where installs kept it before
(`people/`, Serverpod's own session key), so they keep everything; other
instances get folders and keys of their own. What the device shares
across instances (its settings, sounds, the Pomodoro timer's place in the
system) is made once and outlives every switch.

The F-Droid listing drops TetheredNet: the app is no longer tied to the
project's instance.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/l10n/app_localizations_de.dart

@@ -1547,6 +1547,67 @@ class AppLocalizationsDe extends AppLocalizations {
   @override
   String get settingsAccount => 'Konto';
 
+  @override
+  String get instanceWelcome => 'Willkommen bei Halkyon Learn';
+
+  @override
+  String get instanceFirstStartSubtitle =>
+      'Wähle die Instanz, auf der deine Lernsets liegen sollen. Jede betreibt jemand; weitere kannst du später hinzufügen.';
+
+  @override
+  String get instanceAddress => 'Adresse der Instanz';
+
+  @override
+  String instanceUseBuiltIn(String name) {
+    return '$name verwenden';
+  }
+
+  @override
+  String get instanceContinue => 'Weiter';
+
+  @override
+  String get instanceAddressRequired => 'Gib die Adresse einer Instanz ein.';
+
+  @override
+  String get instanceAddressInvalid => 'Das ist keine Adresse.';
+
+  @override
+  String instanceUnreachable(String name) {
+    return 'Unter $name antwortet keine Instanz von Halkyon Learn.';
+  }
+
+  @override
+  String instanceOpenFailed(String name) {
+    return 'Die App auf $name konnte nicht geöffnet werden.';
+  }
+
+  @override
+  String get settingsSwitchAccount => 'Konto wechseln';
+
+  @override
+  String get accountsTitle => 'Konten';
+
+  @override
+  String get accountAdd => 'Konto hinzufügen';
+
+  @override
+  String get accountAddSubtitle =>
+      'Wähle die Instanz für das Konto, das du hinzufügst.';
+
+  @override
+  String get accountRemoveTooltip => 'Konto entfernen';
+
+  @override
+  String get accountRemoveTitle => 'Konto entfernen?';
+
+  @override
+  String accountRemoveMessage(String name) {
+    return 'Entfernt das Konto auf $name von diesem Gerät, mit allem, was das Gerät davon hat. Was es synchronisiert hat, bleibt auf $name: Melde dich dort mit GitHub an, um es wiederzuhaben. In ein anonymes Konto kann man sich nicht wieder anmelden.';
+  }
+
+  @override
+  String get accountRemoveConfirm => 'Entfernen';
+
   @override
   String get settingsSignOut => 'Abmelden';