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>

PROGRESS.md

@@ -158,7 +158,7 @@ Restructure the 3-tab editor panel for better discoverability. Replace Settings
 | 1-4 | Data model + Site tab skeleton | 3h | done |
 | 5-7 | Announcement bar (editable text, link, styles) | 1.5h | done |
 | 8-9 | Social links editor (CRUD, reorder, platform detection, URL normalization) | 2h | done |
-| 10-14 | Header & footer navigation editors | 3h | planned |
+| 10-14 | Header & footer navigation editors | 3h | done |
 | 15-16 | Footer content (about, copyright, newsletter toggle) | 1.25h | done |
 | 17-18 | Move branding from Theme to Site | 1.5h | planned |
 | 19-20 | Merge page settings into Page tab, remove Settings tab | 1h | planned |
@@ -166,6 +166,8 @@ Restructure the 3-tab editor panel for better discoverability. Replace Settings
 
 Social links now support 40+ platforms (grouped by category), auto-detect platform from pasted URLs (including deep links like `tg://` and `spotify:`), normalize bare domains to https://, preserve custom protocols, and filter empty URLs from shop display.
 
+Navigation editors support add/edit/delete/reorder for both header and footer nav items. Page picker dropdown allows linking to system pages (home, about, contact, etc.) or custom published pages. Items persist immediately on change.
+
 ### Unified editor session ([plan](docs/plans/unified-editor-session.md)) — Complete
 
 Unified "optimistic preview with explicit save" across all editor tabs. Changes show immediately but only persist on Save. Free tab switching without warnings. Navigation blocking with Save/Discard/Cancel modal.