feat: Replace custom navbar with mdbook's native navigation

Dropped the custom HTML navbar in favor of mdbook's built-in navigation.
This also resolved an issue where relative anchor links were incorrectly
appearing as entries in the mdbook sidebar.

theme/sepia.css

@@ -1,75 +1,4 @@
 /* Sepia theme override for mdbook */
-/* This makes the mdbook-generated blog match the rest of the site */
-
-/* Navbar overrides - ensure navbar looks identical to main site */
-.navbar {
-    background-color: #5d4037 !important;
-    box-shadow: 0 2px 4px rgba(62, 39, 35, 0.3) !important;
-    position: fixed !important;
-    top: 0 !important;
-    left: 0 !important;
-    right: 0 !important;
-    z-index: 200 !important;
-    border-bottom: 2px solid #4e342e !important;
-    margin: 0 !important;
-    padding: 0 !important;
-}
-
-/* Push mdbook chrome below the fixed navbar */
-.sidebar {
-    top: 3.5rem !important;
-}
-
-.page-wrapper {
-    padding-top: 3.5rem !important;
-}
-
-.nav-container {
-    max-width: 1200px !important;
-    margin: 0 auto !important;
-    padding: 1rem 2rem !important;
-    display: flex !important;
-    justify-content: space-between !important;
-    align-items: center !important;
-}
-
-.nav-brand {
-    color: #f4ecd8 !important;
-    text-decoration: none !important;
-    font-size: 1.5rem !important;
-    font-weight: bold !important;
-}
-
-.nav-brand:hover {
-    color: #d7ccc8 !important;
-}
-
-.nav-menu {
-    list-style: none !important;
-    display: flex !important;
-    gap: 2rem !important;
-    margin: 0 !important;
-    padding: 0 !important;
-}
-
-.nav-menu li {
-    margin: 0 !important;
-    padding: 0 !important;
-    line-height: 1.5 !important;
-}
-
-.nav-menu a {
-    color: #f4ecd8 !important;
-    text-decoration: none !important;
-    font-size: 1rem !important;
-    line-height: 1.5 !important;
-    transition: color 0.3s ease !important;
-    display: inline-block !important;
-}
-
-.nav-menu a:hover {
-    color: #d7ccc8 !important;
-}
 
 :root {
     --bg: #f4ecd8;