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>

lib/berrypod_web/router.ex

@@ -148,7 +148,6 @@ defmodule BerrypodWeb.Router do
     post "/account/totp/cancel", AccountController, :cancel_totp_setup
     get "/account/totp/complete", AccountController, :complete_totp_setup
     post "/account/totp/dismiss-codes", AccountController, :clear_backup_codes
-    post "/navigation", NavigationController, :save
     post "/providers", ProvidersController, :create
     post "/providers/:id", ProvidersController, :update
 
@@ -176,7 +175,6 @@ defmodule BerrypodWeb.Router do
       live "/pages/new", Admin.Pages.CustomForm, :new
       live "/pages/:slug/settings", Admin.Pages.CustomForm, :edit
       live "/pages/:slug", Admin.Pages.Editor, :edit
-      live "/navigation", Admin.Navigation, :index
       live "/media", Admin.Media, :index
       live "/newsletter", Admin.Newsletter, :index
       live "/newsletter/campaigns/new", Admin.Newsletter.CampaignForm, :new