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/package-release.sh

@@ -0,0 +1,48 @@
+#!/usr/bin/env bash
+# Builds, packages, and updates manifest.json for a tagged release. Used by
+# .github/workflows/release.yml; safe to run locally too for a dry run.
+#
+# Required env vars:
+#   VERSION      -- e.g. 0.2.0.0 (must match build.yaml's version)
+#   SERVER_URL   -- e.g. https://gitea.mrcynic.site
+#   REPO_OWNER   -- Gitea org/user the repo lives under
+#   REPO_NAME    -- Gitea repo name
+set -euo pipefail
+
+: "${VERSION:?VERSION env var required, e.g. 0.2.0.0}"
+: "${SERVER_URL:?SERVER_URL env var required, e.g. https://gitea.mrcynic.site}"
+: "${REPO_OWNER:?REPO_OWNER env var required}"
+: "${REPO_NAME:?REPO_NAME env var required}"
+
+ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+cd "$ROOT"
+
+echo "Building Release configuration..."
+dotnet build src/JellyfinSyncPlus/JellyfinSyncPlus.csproj -c Release
+
+BUILD_VERSION=$(grep -oP '(?<=^version: ")[^"]*' build.yaml)
+if [ "$BUILD_VERSION" != "$VERSION" ]; then
+  echo "ERROR: build.yaml version ($BUILD_VERSION) does not match requested VERSION ($VERSION)." >&2
+  echo "Update build.yaml's version field to match the git tag before releasing." >&2
+  exit 1
+fi
+
+mkdir -p release
+ZIP_NAME="JellyfinSyncPlus_${VERSION}.zip"
+ZIP_PATH="release/${ZIP_NAME}"
+rm -f "$ZIP_PATH"
+(cd src/JellyfinSyncPlus/bin/Release/net9.0 && zip -j "$ROOT/$ZIP_PATH" JellyfinSyncPlus.dll)
+
+CHECKSUM=$(md5sum "$ZIP_PATH" | awk '{print $1}')
+SOURCE_URL="${SERVER_URL}/${REPO_OWNER}/${REPO_NAME}/releases/download/v${VERSION}/${ZIP_NAME}"
+
+python3 "$ROOT/dev/update_manifest.py" \
+  --build-yaml "$ROOT/build.yaml" \
+  --manifest "$ROOT/manifest.json" \
+  --version "$VERSION" \
+  --checksum "$CHECKSUM" \
+  --source-url "$SOURCE_URL"
+
+echo "Packaged $ZIP_PATH"
+echo "Checksum: $CHECKSUM"
+echo "Source URL: $SOURCE_URL"