feat: synced lyrics view in Now Playing overlay

Adds a Lyrics tab to the NP overlay using Navidrome's getLyricsBySongId
endpoint. Synced lyrics auto-scroll and highlight the current line as
the track plays; tapping a line seeks to that position. Falls back to
plain text for unsynced lyrics.

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

index.html

@@ -557,6 +557,7 @@
       </button>
       <div class="np-tabs">
         <button class="np-tab active" id="np-tab-player" onclick="switchNpTab('player')">Jetzt läuft</button>
+        <button class="np-tab" id="np-tab-lyrics" onclick="switchNpTab('lyrics')">Lyrics</button>
         <button class="np-tab" id="np-tab-queue" onclick="switchNpTab('queue')">Warteschlange</button>
       </div>
       <button class="np-menu-btn" onclick="openNpMenu()" title="Optionen">
@@ -624,6 +625,10 @@
     </div>
     </div><!-- /np-player-view -->
 
+    <div id="np-lyrics-view">
+      <div id="np-lyrics-content"></div>
+    </div>
+
     <div id="np-queue-view">
       <div id="np-queue-list"></div>
     </div>