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/page_renderer_test.exs

@@ -49,17 +49,17 @@ defmodule BerrypodWeb.PageRendererTest do
     test "home page renders hero and featured products" do
       html = render_page("home", %{products: []})
 
-      assert html =~ "Original designs, printed on demand"
+      assert html =~ "Your headline goes here"
       assert html =~ "Shop the collection"
       assert html =~ "Featured products"
-      assert html =~ "Made with passion, printed with care"
+      assert html =~ "Your story in a nutshell"
     end
 
     test "about page renders hero and content area" do
       html =
         render_page("about", %{content_blocks: [%{type: :paragraph, text: "Test about text"}]})
 
-      assert html =~ "About the studio"
+      assert html =~ "About us"
       assert html =~ "content-body"
     end