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>

src/state.js

@@ -18,6 +18,7 @@ export const S = {
   // Now Playing overlay
   npOpen: false,
   npQueueMode: false,
+  npLyricsMode: false,
 
   // Navigation history
   _navStack: [],