perf: add command sequence numbers, episode-aware sync guard, fix echo suppression

- Add monotonically increasing seq per peer (localSeq + lastSeqBySender)
- Server relays seq field for stale command detection (backward compatible)
- Replace expectedEvents (1500ms timeout) with _suppressTimers (per-type, 300ms)
- Fix FORCE_SYNC_ACK missing seq (stale-ACK could trigger premature exec)
- Fix episode lobby readyPeers asymmetry (initiator never broadcasted readiness)
- Add extractEpisodeId() parsing S01E01/Season 1 Episode 1/Folge 5 patterns
- Add isDifferentEpisode() guard blocking sync commands across different episodes
- add sameEpisode() for format-tolerant lobby title matching
- Guard controlled by autoSyncNextEpisode setting with disable hint in warning
- Reduce reactive lock from 1000ms to 300ms (seq numbers handle ordering)

server/index.js

@@ -454,6 +454,7 @@ io.on('connection', (socket) => {
                     // --- S-3: Construct clean relay payload — never forward raw client data ---
                     const relayPayload = {
                         senderId:        mapping.peerId,
+                        seq:             clampNum(data.seq, 0, Number.MAX_SAFE_INTEGER),
                         currentTime:     clampNum(data.currentTime, 0, 86400),
                         targetTime:      clampNum(data.targetTime, 0, 86400),
                         playbackState:   validState(data.playbackState),