add nav editors to Site tab with live preview

- Add header and footer nav editors to Site tab with drag-to-reorder,
  add/remove items, and destination picker (pages, collections, external)
- Live preview updates as you edit nav items
- Remove legacy /admin/navigation page and controller (was saving to
  Settings table, now uses nav_items table)
- Update error_html.ex and pages/editor.ex to load nav from nav_items table
- Update link_scanner to read from nav_items table, edit path now /?edit=site
- Add Site.default_header_nav/0 and default_footer_nav/0 for previews/errors
- Remove fallback logic from theme_hook.ex (database is now source of truth)
- Seed default nav items and social links during setup

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

assets/css/admin/components.css

@@ -3295,21 +3295,47 @@
 
 .site-editor-nav-item {
   display: flex;
-  justify-content: space-between;
-  align-items: center;
-  padding: 0.5rem 0.75rem;
+  gap: 0.5rem;
+  align-items: flex-start;
+  padding: 0.5rem;
   background: var(--t-surface-sunken);
   border-radius: 0.375rem;
   font-size: 0.8125rem;
 }
 
-.site-editor-nav-label {
+.site-editor-nav-item-form {
+  display: flex;
+  flex-direction: column;
+  gap: 0.375rem;
+  flex: 1;
+  min-width: 0;
+}
+
+.site-editor-nav-label-input {
   font-weight: 500;
 }
 
-.site-editor-nav-url {
-  font-size: 0.75rem;
-  opacity: 0.6;
+.site-editor-nav-url-row {
+  display: flex;
+  gap: 0.375rem;
+}
+
+.site-editor-nav-type {
+  width: 6rem;
+  flex-shrink: 0;
+}
+
+.site-editor-nav-page,
+.site-editor-nav-url-input {
+  flex: 1;
+  min-width: 0;
+}
+
+.site-editor-nav-item-actions {
+  display: flex;
+  gap: 0.125rem;
+  flex-shrink: 0;
+  padding-top: 0.25rem;
 }
 
 /* Social links editor item */