fix: icon permissions and GTK single instance

- Use GTK default flags for single instance
- Fix icon path to absolute /usr/share/icons
- Add postinstall chmod 644 for icon
- Remove Dart-level single instance code

lib/main.dart

@@ -1,70 +1,9 @@
-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);