Add account switcher

Fixes #146

lib/controllers/accounts.dart

@@ -0,0 +1,60 @@
+import "dart:convert";
+
+import "package:fast_immutable_collections/fast_immutable_collections.dart";
+import "package:flutter_riverpod/flutter_riverpod.dart";
+import "package:nexus/controllers/key.dart";
+import "package:nexus/models/account.dart";
+
+class AccountsController extends AsyncNotifier<IList<Account>> {
+  @override
+  Future<IList<Account>> build() async {
+    final stored = await ref.watch(
+      KeyController.provider(KeyController.accountsKey).future,
+    );
+    final accounts = IList<Account>(
+      (json.decode(stored ?? "[]") as List).map(
+        (account) => .fromJson(account),
+      ),
+    );
+
+    // Installs from before account switching are logged into the legacy slot
+    return accounts.isEmpty
+        ? .new([const .new(id: Account.legacyId)])
+        : accounts;
+  }
+
+  Future<void> _set(IList<Account> accounts) async {
+    state = .data(accounts);
+    await ref
+        .read(KeyController.provider(KeyController.accountsKey).notifier)
+        .set(json.encode(accounts.map((account) => account.toJson()).toList()));
+  }
+
+  Future<IList<Account>> get _current async {
+    await future;
+    return state.requireValue;
+  }
+
+  Future<void> save(Account account) async {
+    final accounts = await _current;
+    final index = accounts.indexWhere((existing) => existing.id == account.id);
+
+    await _set(
+      index == -1 ? accounts.add(account) : accounts.replace(index, account),
+    );
+  }
+
+  Future<void> reset(String id) async {
+    final accounts = await _current;
+    final index = accounts.indexWhere((account) => account.id == id);
+    if (index != -1) await _set(accounts.replace(index, .new(id: id)));
+  }
+
+  Future<void> remove(String id) async =>
+      _set((await _current).removeWhere((account) => account.id == id));
+
+  static final provider =
+      AsyncNotifierProvider<AccountsController, IList<Account>>(
+        AccountsController.new,
+      );
+}