feat: add default cover image for all media types

Added a beautiful default cover image featuring Naomi reading in her cozy
library, which will be displayed whenever a media item doesn't have a cover
image provided.

Changes:
- Added default-cover.jpg to frontend public/assets directory
- Updated all list components (games, books, music, shows, manga, art) to
  always display an image using the default as fallback
- Updated all detail components to always show cover section with default
  fallback
- Removed conditional rendering of cover images - now always visible
- Properly handles different property names (coverImage, coverArt, imageUrl)

Benefits:
- Consistent visual appearance across all media types
- No more empty spaces where cover images would be
- Better UX with uniform card layouts
- Beautiful placeholder that matches the library theme

Co-Authored-By: Naomi Carrigan <commits@nhcarrigan.com>

apps/frontend/src/app/components/art/art-gallery.component.ts

@@ -395,7 +395,7 @@ import { Art, CreateArtDto, UpdateArtDto, Comment, SuggestionEntity, Link } from
               <a [routerLink]="['/art', art.id]" class="card-link">
                 <div class="art-image-container">
                   <img
-                    [src]="art.imageUrl"
+                    [src]="art.imageUrl || '/assets/default-cover.jpg'"
                     [alt]="art.description || art.title"
                     class="art-image"
                   >