add Site context with social links editor and site-wide settings

- Add Site context for managing site-wide content (social links, nav items,
  announcement bar, footer content)
- Add SocialLink schema with URL normalization and platform auto-detection
  supporting 40+ platforms via host and 25+ via URI scheme
- Add NavItem schema for header/footer navigation (editor UI coming next)
- Add SiteEditor component with collapsible sections for each content type
- Wire social links card block and footer to use database data
- Filter empty URLs from display in shop components
- Add DetailsPreserver hook to preserve collapsible section state
- Add comprehensive tests for Site context and SocialLink functions
- Remove unused helper functions from onboarding to fix compiler warnings
- Move sync_edit_url_param helper to group handle_editor_event clauses

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

priv/repo/seeds.exs

@@ -10,7 +10,7 @@
 # We recommend using the bang functions (`insert!`, `update!`
 # and so on) as they will fail if something goes wrong.
 
-alias Berrypod.Settings
+alias Berrypod.{Settings, Site}
 
 # Set default theme settings (Studio preset)
 IO.puts("Setting up default theme settings...")
@@ -18,3 +18,10 @@ IO.puts("Setting up default theme settings...")
 {:ok, _theme} = Settings.apply_preset(:studio)
 
 IO.puts("✓ Default theme settings applied (Studio preset)")
+
+# Seed default navigation and social links
+IO.puts("Setting up default site content...")
+
+Site.seed_defaults()
+
+IO.puts("✓ Default navigation and social links created")