Make the progress bar purely decorative, drop live position tracking

positionMs changes every poll (~20s), so the old live progress bar/elapsed
time forced a full flash constantly, even mid-song -- this panel has no
partial refresh, so there's no way to show real progress without that.
Now it's an outline + total length only, driven by nothing that changes
while the same track keeps playing. Verified the rendered image is
byte-identical across different positionMs values for the same track.

Also dropped elapsed/elapsed_text/progress_pct from the track dict
entirely now that nothing renders them.

web/templates/settings_navidrome.html

@@ -25,7 +25,7 @@
 
   <div class="card">
     <div class="section-label">Display options</div>
-    <label class="checkbox"><input type="checkbox" name="show_progress" {{ "checked" if settings.show_progress }}> Progress bar and times</label>
+    <label class="checkbox"><input type="checkbox" name="show_progress" {{ "checked" if settings.show_progress }}> Progress bar (decorative -- shows length, not live position, so it won't force extra refreshes)</label>
     <label class="checkbox"><input type="checkbox" name="show_genre" {{ "checked" if settings.show_genre }}> Genre</label>
     <label class="checkbox"><input type="checkbox" name="show_source" {{ "checked" if settings.show_source }}> Player and listener name</label>
   </div>