feat: Auto-Sync Next Episode v1.2.0

Adds a new toggleable feature that detects episode transitions via
mediaTitle mutation (loadeddata/MutationObserver), pauses the video,
and waits for all room peers to load the same episode before
executing a coordinated Force Sync play at 0:00.

Protocol:
- Add EPISODE_LOBBY and EPISODE_READY events to shared/constants.js
- Add EPISODE_LOBBY_TIMEOUT (60s) constant
- Relay both new events in server/index.js

Content Script (content.js):
- Layered detection: loadeddata + MutationObserver src-change + heartbeat
- Debounced onEpisodeTransition() sends signal ONLY; no eager pause
- PAUSE_FOR_LOBBY handler pauses only after background confirms feature enabled
- startLobbyPoll() polls title match without premature pause for non-initiators
- checkAndReportLobbyReady() pauses and sends EPISODE_READY_LOCAL on match
- CONTENT_BOOT recovery for re-injection after hard navigation

Background (background.js):
- Episode lobby state persisted in chrome.storage.session with recovery
- EPISODE_CHANGED: checks setting, creates lobby, sends PAUSE_FOR_LOBBY to tab
- EPISODE_LOBBY/READY server event handlers with dedup logic
- 60s timeout cancels lobby (Option B) with Chrome failure notification
- Peer departure handled: removes from readyPeers, re-checks completion
- executeEpisodeLobby() reuses existing Force Sync pipeline at targetTime 0.0
- Lobby cleared on LEAVE_ROOM; status exposed in GET_STATUS

Popup:
- Auto-Sync Next Episode toggle in Settings tab (default: off, opt-in)
- Episode Lobby status card in Sync tab with peer readiness display
- LOBBY_UPDATE message handler for real-time UI updates

Bumps APP_VERSION and manifest to 1.2.0

shared/constants.js

@@ -7,7 +7,7 @@
  */
 
 export const PROTOCOL_VERSION = "1.0.0";
-export const APP_VERSION = "1.1.4";
+export const APP_VERSION = "1.2.0";
 
 export const OFFICIAL_SERVER_URL = 'wss://sync.shik3i.net';
 export const OFFICIAL_LANDING_PAGE_URL = 'https://koalasync.shik3i.net';
@@ -32,8 +32,13 @@ export const EVENTS = {
     FORCE_SYNC_EXECUTE: "force_sync_execute",
     EVENT_ACK: "event_ack",
     GET_ROOMS: "get_rooms",
-    ROOM_LIST: "room_list"
+    ROOM_LIST: "room_list",
+
+    // Episode Auto-Sync
+    EPISODE_LOBBY: "episode_lobby",     // Broadcast: waiting for everyone on this episode
+    EPISODE_READY: "episode_ready"      // Response: loaded the episode and paused at 0:00
 };
 
 export const HEARTBEAT_INTERVAL = 15000; // 15s
 export const FORCE_SYNC_TIMEOUT = 5000; // 5s timeout for ACKs
+export const EPISODE_LOBBY_TIMEOUT = 60000; // 60s timeout for episode lobby