feat: add branding assets and favicon support across admin-web and desktop-client

Add NexaVPN logo images (full logo and mark-only variants) to admin-web and desktop-client public directories. Add favicon.ico and favicon.png to admin-web, and icon.png to desktop-client. Update index.html files to reference favicon assets. Add icon.png and icon.ico to desktop-client Tauri icons directory and configure bundle.icon in tauri.conf.json. Update Layout component to display logo in sidebar brand-block with

desktop-client/src-tauri/tauri.conf.json

@@ -25,7 +25,10 @@
   "bundle": {
     "active": true,
     "targets": "all",
-    "icon": [],
+    "icon": [
+      "icons/icon.png",
+      "icons/icon.ico"
+    ],
     "resources": [
       "bundled/**/*"
     ]