feat(playback): move a session between devices

One device holds the audio; the rest watch the same session over an
event stream and act as remotes. Picking a device hands the audio over
at the position the previous one reported, and that device stops.

Also centre the command palette with margins instead of a translate:
animate-rise sets its own transform and dropped the offset on mobile.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/nginx.conf.template

@@ -20,6 +20,20 @@ server {
         proxy_cache_bypass $http_upgrade;
     }
 
+    # The cross-device event stream is long-lived and must arrive unbuffered:
+    # with proxy buffering on, nginx holds each event until it has a chunk worth
+    # forwarding, which is exactly the latency this channel exists to avoid.
+    location /api/playback/stream {
+        proxy_pass http://backend:3000;
+        proxy_http_version 1.1;
+        proxy_set_header Host $host;
+        proxy_set_header Authorization "Bearer ${MUZICK_API_KEY}";
+        proxy_set_header Connection '';
+        proxy_buffering off;
+        proxy_cache off;
+        proxy_read_timeout 1h;
+    }
+
     location /api {
         proxy_pass http://backend:3000;
         proxy_http_version 1.1;