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>

assets/css/admin/components.css

@@ -1958,10 +1958,11 @@
 
 .block-card-info {
   flex: 1;
-  min-width: 0;
+  min-width: 6rem;
 }
 
 .block-card-name {
+  display: block;
   font-size: 0.875rem;
   font-weight: 500;
   white-space: nowrap;
@@ -1983,6 +1984,7 @@
   align-items: center;
   gap: 0.125rem;
   flex-shrink: 0;
+  margin-left: auto;
 }
 
 .block-remove-btn {
@@ -2119,7 +2121,8 @@
 .block-card-header {
   display: flex;
   align-items: center;
-  gap: 0.5rem;
+  gap: 0.25rem 0.5rem;
+  flex-wrap: wrap;
   width: 100%;
 }