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>

build.yaml

@@ -0,0 +1,21 @@
+name: "JellyfinSyncPlus"
+guid: "267dbfe9-bb9c-4eeb-97aa-f0449283cfe6"
+version: "0.2.0.0"
+targetAbi: "10.11.6.0"
+framework: "net9.0"
+owner: "cynic"
+overview: "SyncPlay stats for nerds: live drift/playback view plus a manual one-shot sync button."
+description: >
+  Read-only live view of every active SyncPlay group's members -- drift,
+  position, play method, bitrate, and codec info -- plus a manual "sync me
+  to group" button for one-shot self-correction. Does not touch playback
+  automatically: earlier attempts at automatic quality-forcing and
+  automatic drift correction both caused real problems in live use and were
+  reverted (see PLAN.md). Reachable via Dashboard -> Plugins -> JellyfinSyncPlus.
+category: "General"
+artifacts:
+  - "JellyfinSyncPlus.dll"
+changelog: >
+  0.2.0.0: SyncPlay stats-for-nerds page and manual one-shot "sync me to
+  group" button. No automatic playback intervention.
+  0.1.0.0: Phase 0 scaffold. Trivial plugin, no functionality yet.