feat(authentication): add account linking page for anonymous users

- Create a new page for anonymous users to link their account to an email
- Display options for account linking in a modal style
- Show loading indicator and error handling
- Add localization support for page content

Author: fulleni

lib/authentication/view/account_linking_page.dart

@@ -0,0 +1,128 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_bloc/flutter_bloc.dart';
+import 'package:flutter_news_app_mobile_client_full_source_code/authentication/bloc/authentication_bloc.dart';
+import 'package:flutter_news_app_mobile_client_full_source_code/l10n/l10n.dart';
+import 'package:flutter_news_app_mobile_client_full_source_code/router/routes.dart';
+import 'package:go_router/go_router.dart';
+import 'package:ui_kit/ui_kit.dart';
+
+/// {@template account_linking_page}
+/// Displays options for an anonymous user to link their account to an email.
+///
+/// This page is presented modally to indicate it's an in-app process
+/// rather than a full sign-out/sign-in flow.
+/// {@endtemplate}
+class AccountLinkingPage extends StatelessWidget {
+  /// {@macro account_linking_page}
+  const AccountLinkingPage({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    final l10n = AppLocalizationsX(context).l10n;
+    final textTheme = Theme.of(context).textTheme;
+    final colorScheme = Theme.of(context).colorScheme;
+
+    return Scaffold(
+      appBar: AppBar(
+        backgroundColor: Colors.transparent,
+        elevation: 0,
+        leading: IconButton(
+          icon: const Icon(Icons.close),
+          tooltip: MaterialLocalizations.of(context).closeButtonTooltip,
+          onPressed: () {
+            // Navigate back to the account page when close is pressed.
+            // This uses pop() because it's a fullscreenDialog.
+            context.pop();
+          },
+        ),
+      ),
+      body: SafeArea(
+        child: BlocConsumer<AuthenticationBloc, AuthenticationState>(
+          listener: (context, state) {
+            if (state.status == AuthenticationStatus.failure) {
+              ScaffoldMessenger.of(context)
+                ..hideCurrentSnackBar()
+                ..showSnackBar(
+                  SnackBar(
+                    content: Text(state.exception!.toFriendlyMessage(context)),
+                    backgroundColor: colorScheme.error,
+                  ),
+                );
+            }
+          },
+          builder: (context, state) {
+            final isLoading = state.status == AuthenticationStatus.loading;
+
+            return Padding(
+              padding: const EdgeInsets.all(AppSpacing.paddingLarge),
+              child: Center(
+                child: SingleChildScrollView(
+                  child: Column(
+                    mainAxisAlignment: MainAxisAlignment.center,
+                    crossAxisAlignment: CrossAxisAlignment.stretch,
+                    children: [
+                      // --- Icon ---
+                      Padding(
+                        padding: const EdgeInsets.only(bottom: AppSpacing.xl),
+                        child: Icon(
+                          Icons.sync,
+                          size: AppSpacing.xxl * 2,
+                          color: colorScheme.primary,
+                        ),
+                      ),
+                      // --- Headline and Subheadline ---
+                      Text(
+                        l10n.accountLinkingHeadline,
+                        style: textTheme.headlineMedium?.copyWith(
+                          fontWeight: FontWeight.bold,
+                        ),
+                        textAlign: TextAlign.center,
+                      ),
+                      const SizedBox(height: AppSpacing.md),
+                      Text(
+                        l10n.accountLinkingBody,
+                        style: textTheme.bodyLarge?.copyWith(
+                          color: colorScheme.onSurfaceVariant,
+                        ),
+                        textAlign: TextAlign.center,
+                      ),
+                      const SizedBox(height: AppSpacing.xxl),
+
+                      // --- Email Linking Button ---
+                      ElevatedButton.icon(
+                        icon: const Icon(Icons.email_outlined),
+                        onPressed: isLoading
+                            ? null
+                            : () {
+                                // Navigate to the request code page for linking.
+                                context.goNamed(
+                                  Routes.accountLinkingRequestCodeName,
+                                );
+                              },
+                        label: Text(l10n.accountLinkingSendLinkButton),
+                        style: ElevatedButton.styleFrom(
+                          padding: const EdgeInsets.symmetric(
+                            vertical: AppSpacing.md,
+                          ),
+                          textStyle: textTheme.labelLarge,
+                        ),
+                      ),
+
+                      // --- Loading Indicator ---
+                      if (isLoading) ...[
+                        const Padding(
+                          padding: EdgeInsets.only(top: AppSpacing.xl),
+                          child: Center(child: CircularProgressIndicator()),
+                        ),
+                      ],
+                    ],
+                  ),
+                ),
+              ),
+            );
+          },
+        ),
+      ),
+    );
+  }
+}