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

admin-web/src/styles/global.css

@@ -61,6 +61,45 @@ button {
   backdrop-filter: blur(12px);
 }
 
+.auth-brand,
+.brand-block,
+.topbar-brand {
+  display: flex;
+  align-items: center;
+  gap: 16px;
+}
+
+.brand-block {
+  align-items: flex-start;
+}
+
+.brand-copy {
+  display: grid;
+  gap: 6px;
+}
+
+.brand-tagline {
+  margin: 0;
+  color: var(--muted);
+  max-width: 240px;
+  line-height: 1.5;
+}
+
+.brand-logo {
+  display: block;
+  height: auto;
+}
+
+.brand-logo-full {
+  width: min(100%, 220px);
+}
+
+.brand-logo-mark {
+  width: 48px;
+  border-radius: 14px;
+  box-shadow: 0 10px 30px rgba(0, 0, 0, 0.18);
+}
+
 .auth-card label {
   display: grid;
   gap: 8px;
@@ -258,6 +297,18 @@ button {
     border-bottom: 1px solid var(--line);
   }
 
+  .auth-brand,
+  .brand-block,
+  .topbar-brand,
+  .page-header {
+    align-items: flex-start;
+    flex-direction: column;
+  }
+
+  .brand-logo-full {
+    width: min(100%, 200px);
+  }
+
   .grid.two,
   .grid.three {
     grid-template-columns: 1fr;