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/components/Layout.tsx

@@ -14,9 +14,13 @@ export function Layout() {
   return (
     <div className="shell">
       <aside className="sidebar">
-        <div>
-          <p className="eyebrow">NexaVPN</p>
-          <h1>Control Plane</h1>
+        <div className="brand-block">
+          <img className="brand-logo brand-logo-full" src="/NexaVPN_Logo.png" alt="NexaVPN" />
+          <div className="brand-copy">
+            <p className="eyebrow">NexaVPN</p>
+            <h1>Control Plane</h1>
+            <p className="brand-tagline">Remote access orchestration for your private WireGuard edge.</p>
+          </div>
         </div>
         <nav className="nav">
           {items.map(([label, path]) => (
@@ -32,9 +36,12 @@ export function Layout() {
       </aside>
       <main className="content">
         <header className="topbar">
-          <div>
-            <p className="eyebrow">Enterprise WireGuard</p>
-            <h2>Self-hosted VPN management</h2>
+          <div className="topbar-brand">
+            <img className="brand-logo brand-logo-mark" src="/NexaVPN_Logo_Only.png" alt="NexaVPN mark" />
+            <div>
+              <p className="eyebrow">Enterprise WireGuard</p>
+              <h2>Self-hosted VPN management</h2>
+            </div>
           </div>
           <div className="pill">Secure by design</div>
         </header>