fix content image double-suffix, clean up page defaults and editor UX

- Fix resolve_content_image returning base path (not full URL) so
  responsive_image doesn't double-append width/extension
- Remove legacy image fields (image_src, image_alt, image_url) from
  block settings schemas
- Remove demo/mockup fallbacks from renderer and defaults — blank
  fields stay blank instead of showing preview content
- Replace demo text in defaults with instructional placeholders that
  guide new shop owners
- Remove redundant X button from editor sidebar, add unsaved-changes
  confirmation to Done button
- Fix block card name overflow on mobile (display: block, flex-wrap)
- Add onboarding UX improvement plan (10 tasks)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test/berrypod_web/controllers/page_controller_test.exs

@@ -11,6 +11,6 @@ defmodule BerrypodWeb.PageControllerTest do
 
   test "GET / renders the shop home page", %{conn: conn} do
     conn = get(conn, ~p"/")
-    assert html_response(conn, 200) =~ "Original designs, printed on demand"
+    assert html_response(conn, 200) =~ "Your headline goes here"
   end
 end