feat: Implement in-app signup

lib/pages/signup.dart

@@ -0,0 +1,58 @@
+import 'package:fluffychat/pages/views/signup_view.dart';
+import 'package:fluffychat/utils/platform_infos.dart';
+import 'package:fluffychat/widgets/matrix.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_gen/gen_l10n/l10n.dart';
+import '../utils/localized_exception_extension.dart';
+
+class SignupPage extends StatefulWidget {
+  const SignupPage({Key key}) : super(key: key);
+
+  @override
+  SignupPageController createState() => SignupPageController();
+}
+
+class SignupPageController extends State<SignupPage> {
+  final TextEditingController usernameController = TextEditingController();
+  final TextEditingController passwordController = TextEditingController();
+  String usernameError;
+  String passwordError;
+  bool loading = false;
+  bool showPassword = true;
+
+  void toggleShowPassword() => setState(() => showPassword = !showPassword);
+
+  void signup([_]) async {
+    usernameError = passwordError = null;
+
+    if (usernameController.text.isEmpty) {
+      return setState(
+          () => usernameError = L10n.of(context).pleaseChooseAUsername);
+    }
+    if (passwordController.text.isEmpty) {
+      return setState(
+          () => passwordError = L10n.of(context).chooseAStrongPassword);
+    }
+
+    setState(() => loading = true);
+
+    try {
+      final client = Matrix.of(context).client;
+      await client.uiaRequestBackground(
+        (auth) => client.register(
+          username: usernameController.text,
+          password: passwordController.text,
+          initialDeviceDisplayName: PlatformInfos.clientName,
+          auth: auth,
+        ),
+      );
+    } catch (e) {
+      passwordError = (e as Object).toLocalizedString(context);
+    } finally {
+      setState(() => loading = false);
+    }
+  }
+
+  @override
+  Widget build(BuildContext context) => SignupPageView(this);
+}