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/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>