neon: Use theme extension for tablet layout

packages/neon/neon/lib/src/pages/login.dart

@@ -2,6 +2,7 @@ import 'package:flutter/material.dart';
 import 'package:neon/l10n/localizations.dart';
 import 'package:neon/src/models/branding.dart';
 import 'package:neon/src/router.dart';
+import 'package:neon/src/utils/theme.dart';
 import 'package:neon/src/utils/validators.dart';
 import 'package:neon/src/widgets/nextcloud_logo.dart';
 import 'package:provider/provider.dart';
@@ -59,9 +60,7 @@ class _LoginPageState extends State<LoginPage> {
       ),
       body: Center(
         child: ConstrainedBox(
-          constraints: const BoxConstraints(
+          constraints: Theme.of(context).extension<NeonTheme>()?.tabletLayout ?? const BoxConstraints(),
-            maxWidth: 640,
-          ),
           child: Scrollbar(
             interactive: true,
             child: SingleChildScrollView(

packages/neon/neon/lib/src/pages/login_check_account.dart

@@ -6,6 +6,7 @@ import 'package:neon/src/blocs/accounts.dart';
 import 'package:neon/src/blocs/login_check_account.dart';
 import 'package:neon/src/models/account.dart';
 import 'package:neon/src/router.dart';
+import 'package:neon/src/utils/theme.dart';
 import 'package:neon/src/widgets/account_tile.dart';
 import 'package:neon/src/widgets/exception.dart';
 import 'package:neon/src/widgets/linear_progress_indicator.dart';
@@ -57,9 +58,7 @@ class _LoginCheckAccountPageState extends State<LoginCheckAccountPage> {
           child: Padding(
             padding: const EdgeInsets.all(10),
             child: ConstrainedBox(
-              constraints: const BoxConstraints(
+              constraints: Theme.of(context).extension<NeonTheme>()?.tabletLayout ?? const BoxConstraints(),
-                maxWidth: 640,
-              ),
               child: ResultBuilder.behaviorSubject(
                 stream: bloc.state,
                 builder: (final context, final state) => Column(

packages/neon/neon/lib/src/pages/login_check_server_status.dart

@@ -3,6 +3,7 @@ import 'package:neon/l10n/localizations.dart';
 import 'package:neon/src/bloc/result.dart';
 import 'package:neon/src/bloc/result_builder.dart';
 import 'package:neon/src/blocs/login_check_server_status.dart';
+import 'package:neon/src/utils/theme.dart';
 import 'package:neon/src/widgets/exception.dart';
 import 'package:neon/src/widgets/linear_progress_indicator.dart';
 import 'package:neon/src/widgets/validation_tile.dart';
@@ -45,9 +46,7 @@ class _LoginCheckServerStatusPageState extends State<LoginCheckServerStatusPage>
           child: Padding(
             padding: const EdgeInsets.all(10),
             child: ConstrainedBox(
-              constraints: const BoxConstraints(
+              constraints: Theme.of(context).extension<NeonTheme>()?.tabletLayout ?? const BoxConstraints(),
-                maxWidth: 640,
-              ),
               child: ResultBuilder.behaviorSubject(
                 stream: bloc.state,
                 builder: (final context, final state) => Column(

packages/neon/neon/lib/src/utils/theme.dart

@@ -48,7 +48,11 @@ class AppTheme {
       snackBarTheme: _snackBarTheme,
       dividerTheme: _dividerTheme,
       extensions: [
-        const NeonTheme(),
+        const NeonTheme(
+          tabletLayout: BoxConstraints(
+            maxWidth: 640,
+          ),
+        ),
         ...?appThemes,
       ],
     );
@@ -70,16 +74,27 @@ class AppTheme {
 @internal
 @immutable
 class NeonTheme extends ThemeExtension<NeonTheme> {
-  const NeonTheme();
+  const NeonTheme({
+    required this.tabletLayout,
+  });
+
+  final BoxConstraints? tabletLayout;
 
   @override
-  NeonTheme copyWith() => const NeonTheme();
+  NeonTheme copyWith({
+    final BoxConstraints? tabletLayout,
+  }) =>
+      NeonTheme(
+        tabletLayout: tabletLayout ?? this.tabletLayout,
+      );
 
   @override
   NeonTheme lerp(final NeonTheme? other, final double t) {
     if (other is! NeonTheme) {
       return this;
     }
-    return const NeonTheme();
+    return NeonTheme(
+      tabletLayout: BoxConstraints.lerp(tabletLayout, other.tabletLayout, t),
+    );
   }
 }