feat(linux): package Linux release, deploy to server, add in-app update banner

Build task embeds GIT_HASH via --dart-define; new deploy-linux-to-server task
packages a tar.gz and updates latest.json on the server. The account list screen
shows a MaterialBanner when a newer Linux build is available.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/core/services/update_service.dart

@@ -0,0 +1,37 @@
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:http/http.dart' as http;
+
+const _kAppVersion = String.fromEnvironment('GIT_HASH');
+const _kLatestJsonUrl = 'https://sharedinbox.de/latest.json';
+
+class UpdateInfo {
+  const UpdateInfo({required this.latestVersion, required this.downloadUrl});
+
+  final String latestVersion;
+  final String downloadUrl;
+}
+
+/// Returns an [UpdateInfo] when a newer Linux version is available, or null
+/// if the app is up to date, the version is unknown, or the platform is not
+/// Linux desktop.
+final updateInfoProvider = FutureProvider<UpdateInfo?>((ref) async {
+  if (!Platform.isLinux || _kAppVersion.isEmpty) return null;
+
+  try {
+    final resp = await http
+        .get(Uri.parse(_kLatestJsonUrl))
+        .timeout(const Duration(seconds: 10));
+    if (resp.statusCode != 200) return null;
+    final json = jsonDecode(resp.body) as Map<String, dynamic>;
+    final latest = json['version'] as String?;
+    final url = json['linux'] as String?;
+    if (latest == null || url == null) return null;
+    if (latest == _kAppVersion) return null;
+    return UpdateInfo(latestVersion: latest, downloadUrl: url);
+  } catch (_) {
+    return null;
+  }
+});