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

public/assets/logo.svg

@@ -0,0 +1,25 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 220 172" fill="none">
+  <!--
+    NixPanel logo mark
+    N letterform built from horizontal server-rack bars + diagonal stroke
+    Panel indicators (stacked blocks) on the right
+  -->
+
+  <!-- Top-left bar — left vertical of the N -->
+  <polygon points="0,0 108,0 133,38 0,38" fill="#3b82f6"/>
+
+  <!-- N diagonal stroke -->
+  <polygon points="108,0 133,0 98,134 73,134" fill="#3b82f6"/>
+
+  <!-- Bottom bar — right side of the N -->
+  <polygon points="73,134 220,134 220,172 73,172" fill="#3b82f6"/>
+
+  <!-- Small square — bottom-left anchor -->
+  <rect x="0" y="134" width="62" height="38" rx="5" fill="#3b82f6"/>
+
+  <!-- Panel block — top right (largest) -->
+  <rect x="152" y="0" width="68" height="30" rx="5" fill="#3b82f6"/>
+
+  <!-- Panel block — middle right -->
+  <rect x="140" y="38" width="80" height="22" rx="5" fill="#3b82f6"/>
+</svg>