feat: single instance with window activation

- Check for existing process via PID
- Activate existing window with wmctrl
- Remove stale lock files
- Prevent multiple instances opening

lib/main.dart

@@ -1,9 +1,70 @@
+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';
 
+final lockFile = File('/tmp/umbrix.lock');
+
+Future<void> _activateExistingWindow() async {
+  // Try to activate existing window via wmctrl
+  try {
+    final result = await Process.run('wmctrl', ['-a', 'Umbrix']);
+    if (result.exitCode == 0) return;
+  } catch (e) {
+    // wmctrl not available, try xdotool
+    try {
+      await Process.run('xdotool', ['search', '--name', 'Umbrix', 'windowactivate']);
+    } catch (e) {
+      // No window activation possible
+    }
+  }
+}
+
+void _cleanupLockFile() {
+  try {
+    if (lockFile.existsSync()) {
+      lockFile.deleteSync();
+    }
+  } catch (e) {
+    // Ignore cleanup errors
+  }
+}
+
 void main() async {
+  // Single instance check - BEFORE Flutter initialization
+  if (Platform.isLinux || Platform.isWindows) {
+    if (await lockFile.exists()) {
+      // Check if process is still alive
+      try {
+        final pidString = await lockFile.readAsString();
+        final pid = int.tryParse(pidString.trim());
+        if (pid != null) {
+          final result = await Process.run('ps', ['-p', pid.toString()]);
+          if (result.exitCode != 0) {
+            // Process dead, remove stale lock
+            await lockFile.delete();
+          } else {
+            // Process alive - activate window and exit
+            await _activateExistingWindow();
+            exit(0);
+          }
+        }
+      } catch (e) {
+        // Error reading lock, remove it
+        try { await lockFile.delete(); } catch (e) {}
+      }
+    }
+    
+    // Create lock file
+    try {
+      await lockFile.create();
+      await lockFile.writeAsString(pid.toString());
+    } catch (e) {
+      // If can't create, continue anyway
+    }
+  }
+
   final widgetsBinding = WidgetsFlutterBinding.ensureInitialized();
 
   SystemChrome.setEnabledSystemUIMode(SystemUiMode.edgeToEdge);