feat: add logo SVG, favicon, wire logo into header and footer

- public/assets/logo.svg: SVG recreation of the NixPanel N-mark
  (server-rack bars + diagonal stroke + panel indicator blocks)
- head.php: favicon links for SVG, PNG 32px, PNG 192px, Apple touch,
  theme-color meta tag
- header.php: logo image next to wordmark, PNG fallback via onerror
- footer.php: logo image in footer brand

views/layout/footer.php

@@ -3,7 +3,10 @@
     <div class="max-w-6xl mx-auto px-4 sm:px-6 lg:px-8">
         <div class="grid grid-cols-2 md:grid-cols-4 gap-8 mb-10">
             <div class="col-span-2 md:col-span-1">
-                <a href="/" class="text-lg font-bold text-white">Nix<span class="text-brand-500">Panel</span></a>
+                <a href="/" class="flex items-center gap-2">
+                <img src="/assets/logo.svg" alt="NixPanel" class="h-6 w-auto" onerror="this.style.display='none'">
+                <span class="text-lg font-bold text-white">Nix<span class="text-brand-500">Panel</span></span>
+            </a>
                 <p class="mt-2 text-sm text-gray-500 leading-relaxed">The hosting control panel<br>you actually want to use.</p>
             </div>
             <div>

views/layout/head.php

@@ -5,6 +5,12 @@
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title><?= e($pageTitle ?? 'NixPanel — Modern Web Hosting Control Panel') ?></title>
     <meta name="description" content="<?= e($pageDesc ?? 'NixPanel is a modern, open-source cPanel alternative. Manage your Linux server with NixServer and NixClient.') ?>">
+    <!-- Favicon -->
+    <link rel="icon" type="image/svg+xml" href="/assets/logo.svg">
+    <link rel="icon" type="image/png" sizes="32x32" href="/assets/logo-32.png">
+    <link rel="icon" type="image/png" sizes="192x192" href="/assets/logo-192.png">
+    <link rel="apple-touch-icon" href="/assets/logo-192.png">
+    <meta name="theme-color" content="#0f172a">
     <link rel="preconnect" href="https://fonts.googleapis.com">
     <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
     <link href="https://fonts.googleapis.com/css2?family=Inter:wght@400;500;600;700;800&display=swap" rel="stylesheet">

views/layout/header.php

@@ -1,6 +1,7 @@
 <header class="border-b border-gray-800 bg-gray-950/80 backdrop-blur-sm sticky top-0 z-40">
     <div class="max-w-6xl mx-auto px-4 sm:px-6 lg:px-8 h-16 flex items-center justify-between">
-        <a href="/" class="flex items-center gap-2">
+        <a href="/" class="flex items-center gap-2.5">
+            <img src="/assets/logo.svg" alt="NixPanel" class="h-7 w-auto" onerror="this.style.display='none'">
             <span class="text-xl font-bold text-white">Nix<span class="text-brand-500">Panel</span></span>
         </a>