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/shows/shows-list.component.ts

@@ -556,9 +556,7 @@ import { Show, ShowStatus, ShowType, CreateShowDto, UpdateShowDto, Comment, Sugg
           @for (show of paginatedShows(); track show.id) {
             <div class="show-card" [class.completed]="show.status === ShowStatus.completed">
               <a [routerLink]="['/shows', show.id]" class="card-link">
-                @if (show.coverImage) {
-                  <img [src]="show.coverImage" [alt]="show.title" class="show-cover">
-                }
+                <img [src]="show.coverImage || '/assets/default-cover.jpg'" [alt]="show.title" class="show-cover">
 
                 <div class="show-info">
                   <h3>{{ show.title }}</h3>