feat(media): reusable media management

Sprint 22.

- MediaAsset gains folder (flat) and MediaListParams gains folder/tag
  filters; MediaRepository.listFolders() derives the folder list from
  existing records
- upload validation: 10MB size cap, mime allow-list (jpeg/png/webp/gif/
  svg+xml/pdf), real error messages surfaced through MediaLibraryFacade
  instead of a generic swallowed string
- SVG uploads are sanitized (script tags and on*= attributes stripped)
  before storage
- raster images (excl. gif) are downscaled to a 2000px max dimension and
  re-encoded via canvas before storage - compression, not a crop UI
- tag editing (window.prompt, comma-separated) via
  MediaLibraryFacade.updateTags()
- MediaPickerComponent wired into Project Editor branding (logo, compact
  logo, favicon) alongside its existing category/product usage - confirmed
  no image fields exist on Static Pages or as a dedicated hero field to
  wire

docs/ADMIN.md updated with the new Sprint 22 section including the storage
abstraction note (MediaRepository was already the abstraction).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/shared/media/media-picker/media-picker.component.scss

@@ -34,6 +34,12 @@
   padding: 6px;
 }
 
+.media-picker__error {
+  color: var(--color-danger, #d9433c);
+  font-size: 0.85rem;
+  margin: 0;
+}
+
 .media-picker__filename {
   display: block;
   font-size: 0.75rem;