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_en.dart

@@ -1537,6 +1537,67 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get settingsAccount => 'Account';
 
+  @override
+  String get instanceWelcome => 'Welcome to Halkyon Learn';
+
+  @override
+  String get instanceFirstStartSubtitle =>
+      'Choose the instance to keep your studysets on. Someone runs each one; you can add others later.';
+
+  @override
+  String get instanceAddress => 'Instance address';
+
+  @override
+  String instanceUseBuiltIn(String name) {
+    return 'Use $name';
+  }
+
+  @override
+  String get instanceContinue => 'Continue';
+
+  @override
+  String get instanceAddressRequired => 'Enter the address of an instance.';
+
+  @override
+  String get instanceAddressInvalid => 'This isn\'t an address.';
+
+  @override
+  String instanceUnreachable(String name) {
+    return 'No instance of Halkyon Learn answers at $name.';
+  }
+
+  @override
+  String instanceOpenFailed(String name) {
+    return 'Couldn\'t open the app on $name.';
+  }
+
+  @override
+  String get settingsSwitchAccount => 'Switch account';
+
+  @override
+  String get accountsTitle => 'Accounts';
+
+  @override
+  String get accountAdd => 'Add account';
+
+  @override
+  String get accountAddSubtitle =>
+      'Choose the instance for the account to add.';
+
+  @override
+  String get accountRemoveTooltip => 'Remove account';
+
+  @override
+  String get accountRemoveTitle => 'Remove account?';
+
+  @override
+  String accountRemoveMessage(String name) {
+    return 'Removes the account on $name from this device, with all the device holds of it. What it synced stays on $name: sign in there with GitHub to have it again. An anonymous account can\'t be signed into again.';
+  }
+
+  @override
+  String get accountRemoveConfirm => 'Remove';
+
   @override
   String get settingsSignOut => 'Sign out';