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>