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/music/music-detail.component.ts

@@ -46,11 +46,9 @@ import { Music, Comment, MusicStatus, MusicType, UpdateMusicDto } from '@library
         </div>
       } @else if (music()) {
         <div class="music-detail-card">
-          @if (music()!.coverArt) {
-            <div class="music-cover-section">
-              <img [src]="music()!.coverArt" [alt]="music()!.title" class="music-cover-large">
-            </div>
-          }
+          <div class="music-cover-section">
+            <img [src]="music()!.coverArt || '/assets/default-cover.jpg'" [alt]="music()!.title" class="music-cover-large">
+          </div>
 
           <div class="music-content">
             <div class="music-header">