feat: add Google Calendar timetable sync via accounts OAuth

OAuth flows through accounts.betterseqta.org with token exchange server-side. Sync runs in the content script to avoid MV3 service worker timeouts. Adds Svelte calendar sync UI on the timetable page, extension asset URL helper, and excludes calendar tokens from cloud sync.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/background.ts

@@ -13,6 +13,7 @@ import {
   withSuppressedCloudAutoUpload,
 } from "./background/cloudSettingsAutoSync";
 import { isAllowedFetchUrl } from "@/seqta/utils/allowedFetchUrl";
+import { registerGoogleCalendarMessageHandlers } from "./background/googleCalendar";
 
 /**
  * Session-only dev-mode override of the content API base.
@@ -557,6 +558,8 @@ const MESSAGE_HANDLERS: Record<string, MessageHandler> = {
   },
 };
 
+registerGoogleCalendarMessageHandlers(MESSAGE_HANDLERS, isTrustedSender);
+
 browser.runtime.onMessage.addListener(
   // @ts-ignore - OnMessageListener expects literal true for async, we return boolean
   (request: any, sender: browser.Runtime.MessageSender, sendResponse: MessageSender) => {