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_editor_hook_test.exs

@@ -90,10 +90,8 @@ defmodule BerrypodWeb.PageEditorHookTest do
                "button[phx-click='editor_toggle_sidebar'][aria-label='Show editor sidebar']"
              )
 
-      # Close the sidebar via the X button
-      view
-      |> element("button[phx-click='editor_toggle_sidebar'][aria-label='Close sidebar']")
-      |> render_click()
+      # Close the sidebar via the backdrop
+      view |> element(".page-editor-backdrop") |> render_click()
 
       assert has_element?(view, "[data-sidebar-open='false']")
       # Pencil button appears in header to re-open