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>

test/berrypod_web/page_editor_hook_test.exs

@@ -171,7 +171,7 @@ defmodule BerrypodWeb.PageEditorHookTest do
       |> render_click()
 
       # Save
-      view |> element("button[phx-click='editor_save']") |> render_click()
+      view |> element("button[phx-click='editor_save_all']") |> render_click()
 
       # Verify persistence
       updated = Pages.get_page("home")
@@ -277,7 +277,7 @@ defmodule BerrypodWeb.PageEditorHookTest do
       |> element("button[phx-click='editor_move_up'][phx-value-id='#{second_id}']")
       |> render_click()
 
-      view |> element("button[phx-click='editor_save']") |> render_click()
+      view |> element("button[phx-click='editor_save_all']") |> render_click()
 
       # After save, undo should be disabled
       assert has_element?(view, "button[phx-click='editor_undo'][disabled]")