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.html

@@ -14,7 +14,7 @@
       <input
         #fileInput
         type="file"
-        accept="image/*,application/pdf"
+        accept="image/jpeg,image/png,image/webp,image/gif,image/svg+xml,application/pdf"
         hidden
         (change)="onFileSelected($event)"
       />
@@ -23,6 +23,10 @@
       </app-button>
     </div>
 
+    @if (facade.error()) {
+      <p class="media-picker__error">{{ facade.error() }}</p>
+    }
+
     @if (facade.loading()) {
       <div class="media-picker__grid">
         @for (i of [1, 2, 3, 4]; track i) {