ui(player): soften vinyl-stage halo, transparent-bg album placeholder, crossfade artwork swaps

- vinyl-stage background fades to transparent (matches fullscreen Listening Room) — no more rectangular dark card around the sleeve+disc composition
- album-art placeholder SVG drops the opaque #282828 backdrop in favour of a translucent disc glyph so the sleeve cardstock shows through before the first artwork load
- new swapArtworkSrc helper retriggers the .is-swapping keyframes so artwork changes crossfade instead of popping

media_server/static/index.html

@@ -187,7 +187,7 @@
                 <div class="vinyl-stage album-art-container">
                     <img id="album-art-glow" class="album-art-glow" src="data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 300 300'%3E%3Crect fill='%23282828' width='300' height='300'/%3E%3C/svg%3E" alt="" aria-hidden="true">
                     <div class="sleeve">
-                        <img id="album-art" src="data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 300 300'%3E%3Crect fill='%23282828' width='300' height='300'/%3E%3Cpath fill='%236a6a6a' d='M150 80c-38.66 0-70 31.34-70 70s31.34 70 70 70 70-31.34 70-70-31.34-70-70-70zm0 20c27.614 0 50 22.386 50 50s-22.386 50-50 50-50-22.386-50-50 22.386-50 50-50zm0 30a20 20 0 100 40 20 20 0 000-40z'/%3E%3C/svg%3E" alt="Album Art">
+                        <img id="album-art" src="data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 300 300'%3E%3Cpath fill='%236a6a6a' opacity='0.35' d='M150 80c-38.66 0-70 31.34-70 70s31.34 70 70 70 70-31.34 70-70-31.34-70-70-70zm0 20c27.614 0 50 22.386 50 50s-22.386 50-50 50-50-22.386-50-50 22.386-50 50-50zm0 30a20 20 0 100 40 20 20 0 000-40z'/%3E%3C/svg%3E" alt="Album Art">
                         <div class="sleeve-grain" aria-hidden="true"></div>
                         <div class="sleeve-corner" aria-hidden="true"></div>
                     </div>