fix: single instance with lock file only

lib/main.dart

@@ -1,9 +1,44 @@
+import 'dart:io';
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:umbrix/bootstrap.dart';
 import 'package:umbrix/core/model/environment.dart';
 
 void main() async {
+  // Single instance check - BEFORE Flutter initialization
+  if (Platform.isLinux || Platform.isWindows) {
+    final lockFile = File('/tmp/umbrix.lock');
+    
+    if (await lockFile.exists()) {
+      try {
+        final pidString = await lockFile.readAsString();
+        final pid = int.tryParse(pidString.trim());
+        
+        if (pid != null) {
+          // Check if process is still alive
+          final result = await Process.run('ps', ['-p', pid.toString()]);
+          
+          if (result.exitCode == 0) {
+            // Process alive - just exit without trying to activate
+            print('Umbrix уже запущен');
+            exit(0);
+          } else {
+            // Stale lock - remove it
+            await lockFile.delete();
+          }
+        }
+      } catch (e) {
+        try { await lockFile.delete(); } catch (_) {}
+      }
+    }
+    
+    // Create lock file with current PID
+    try {
+      await lockFile.create();
+      await lockFile.writeAsString(pid.toString());
+    } catch (_) {}
+  }
+
   final widgetsBinding = WidgetsFlutterBinding.ensureInitialized();
 
   SystemChrome.setEnabledSystemUIMode(SystemUiMode.edgeToEdge);