Respect base URL for nav logo; prefer logo.svg over favicon.svg

Reference the navbar brand image through relURL so it resolves
correctly when the site is served from a non-root base URL. Use
static/img/logo.svg when present, falling back to img/favicon.svg.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

layouts/_partials/header.html

@@ -1,7 +1,9 @@
 <nav class="navbar navbar-expand-lg navbar-light bg-light sticky-top">
   <div class="container"> <!-- Changed from container-fluid to container -->
     <a class="navbar-brand fw-bold" href="{{ .Site.BaseURL }}">
-      <img src="/img/favicon.svg" alt="{{ .Site.Title }}" title="{{ .Site.Title }}">
+      {{- $logo := "img/favicon.svg" -}}
+      {{- if os.FileExists "static/img/logo.svg" -}}{{- $logo = "img/logo.svg" -}}{{- end -}}
+      <img src="{{ $logo | relURL }}" alt="{{ .Site.Title }}" title="{{ .Site.Title }}">
     </a>
     
     <button class="navbar-toggler" type="button" data-bs-toggle="collapse"