Web player served by stemserve

Plex browsing, stem mixer with presets, shuffle, repeat all/one, playback
speed, queue, waveform seek. stemserve serves web/ at / without the token.
stemserve image now builds from the repo root.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/docker-compose.yml

@@ -1,6 +1,6 @@
 services:
   stemserve:
-    build: .
+    build: {context: .., dockerfile: server/Dockerfile}
     image: stemserve:latest
     container_name: stemserve
     restart: unless-stopped