Add SyncPlay stats-for-nerds page, manual sync button, and release pipeline

Phase 0 scaffold plus everything since: quality negotiation, host-relay,
and active drift correction were each built and reverted after real bugs
in live use (see PLAN.md for the full account of each). What remains is
read-only SyncPlay observability (live drift/playback stats page) and a
one-shot manual "sync me to group" action, plus Phase 5 packaging/release
tooling (build.yaml, manifest.json generator, Gitea Actions release
workflow) to distribute it as a proper plugin repository.

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

dev/docker-compose.yml

@@ -0,0 +1,18 @@
+services:
+  jellyfin-dev:
+    image: linuxserver/jellyfin:10.11.6
+    container_name: jellyfinsyncplus-dev
+    environment:
+      - PUID=1000
+      - PGID=1000
+      - TZ=Etc/UTC
+    ports:
+      - "8096:8096"
+    volumes:
+      - ./config:/config
+      - ./plugins/JellyfinSyncPlus_0.1.0.0:/config/data/plugins/JellyfinSyncPlus_0.1.0.0
+      # Real video content for testing Phase 2 relay. Read-only: recordings here are
+      # never to be deleted or modified by this project, mounting :ro enforces that
+      # rather than just relying on discipline.
+      - /mnt/geforce:/media/geforce:ro
+    restart: "no"