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/migrations/20260327163916_create_social_links.exs

@@ -0,0 +1,16 @@
+defmodule Berrypod.Repo.Migrations.CreateSocialLinks do
+  use Ecto.Migration
+
+  def change do
+    create table(:social_links, primary_key: false) do
+      add :id, :binary_id, primary_key: true
+      add :platform, :string, null: false
+      add :url, :string, null: false
+      add :position, :integer, null: false, default: 0
+
+      timestamps(type: :utc_datetime)
+    end
+
+    create index(:social_links, [:position])
+  end
+end