feat: add album artist field to metadata editor

Adds an editable Album-Künstler input to the info modal. Populated from
Navidrome's albumArtist field on open; saved as album_artist tag via
ffmpeg. Explicit value takes precedence; falls back to deriving from
artist (first semicolon segment) when artist is changed without an
explicit albumArtist.

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

index.html

@@ -917,6 +917,7 @@
       <div class="info-edit-fields">
         <label class="info-edit-label">Titel<input class="info-edit-input" id="ie-title" type="text" placeholder="Titel"></label>
         <label class="info-edit-label">Künstler<input class="info-edit-input" id="ie-artist" type="text" placeholder="Künstler (mit ; trennen)"></label>
+        <label class="info-edit-label">Album-Künstler<input class="info-edit-input" id="ie-albumartist" type="text" placeholder="Album-Künstler (mit , trennen)"></label>
         <label class="info-edit-label">Album<input class="info-edit-input" id="ie-album" type="text" placeholder="Album"></label>
         <label class="info-edit-label">Genre<input class="info-edit-input" id="ie-genre" type="text" placeholder="Genre"></label>
         <div style="display:grid;grid-template-columns:1fr 1fr;gap:8px">