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/features/per_app_proxy/overview/per_app_proxy_page.dart

@@ -67,16 +67,16 @@ class PerAppProxyPage extends HookConsumerWidget with PresLogger {
             tooltip: localizations.searchFieldLabel,
           ),
       ],
-      bottom: PlatformUtils.isDesktop 
-        ? null // На Desktop только вкладка "Домены"
-        : TabBar(
-            controller: tabController,
-            onTap: (index) => currentTab.value = index,
-            tabs: [
-              Tab(text: t.settings.network.excludedDomains.domainsTab),
-              Tab(text: t.settings.network.excludedDomains.appsTab),
-            ],
-          ),
+      bottom: PlatformUtils.isDesktop
+          ? null // На Desktop только вкладка "Домены"
+          : TabBar(
+              controller: tabController,
+              onTap: (index) => currentTab.value = index,
+              tabs: [
+                Tab(text: t.settings.network.excludedDomains.domainsTab),
+                Tab(text: t.settings.network.excludedDomains.appsTab),
+              ],
+            ),
     );
 
     final searchAppBar = SliverAppBar(