feat: all-songs view as separate page, iTunes album detail page

- 'Alle X Songs' button now navigates to a dedicated page instead of expanding inline
- Partial/not-downloaded albums now open an iTunes album detail page with track list,
  preview buttons and download option
- Added page-artist-all-songs and page-itunes-album-detail to index.html
- Replaced _expandArtistAllSongs with loadArtistAllSongs + loadItunesAlbumDetail

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

index.html

@@ -177,6 +177,31 @@
         <div id="artist-albums-section"></div>
       </div>
 
+      <!-- ARTIST ALL SONGS -->
+      <div class="page" id="page-artist-all-songs">
+        <button class="back-btn" id="artist-songs-back-btn"><svg width="13" height="13" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><polyline points="15 18 9 12 15 6"/></svg>Zurück</button>
+        <div class="page-header" style="margin-top:8px"><div class="page-title" id="artist-songs-title">Alle Songs</div></div>
+        <div id="artist-songs-list"></div>
+      </div>
+
+      <!-- ITUNES ALBUM DETAIL -->
+      <div class="page" id="page-itunes-album-detail">
+        <button class="back-btn" id="itunes-album-back-btn"><svg width="13" height="13" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><polyline points="15 18 9 12 15 6"/></svg>Zurück</button>
+        <div class="album-header">
+          <div class="album-cover" id="itunes-album-cover-wrap"><div class="no-art">💿</div></div>
+          <div class="album-meta">
+            <div class="album-type">iTunes</div>
+            <div class="album-name" id="itunes-album-detail-name">—</div>
+            <div class="album-artist-name" id="itunes-album-detail-artist"></div>
+            <div class="album-stats" id="itunes-album-detail-stats"></div>
+            <div class="album-actions">
+              <button class="btn-outline" id="itunes-album-dl-btn"><svg width="13" height="13" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><path d="M21 15v4a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2v-4"/><polyline points="7 10 12 15 17 10"/><line x1="12" y1="15" x2="12" y2="3"/></svg>Album laden</button>
+            </div>
+          </div>
+        </div>
+        <div id="itunes-album-songs"></div>
+      </div>
+
       <!-- ALBUMS -->
       <div class="page" id="page-albums">
         <div class="page-header"><div class="page-title">Alben</div></div>