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/seqta/utils/cloudSettingsSync.ts

@@ -37,6 +37,7 @@ export const KEYS_OMITTED_FROM_CLOUD_UPLOAD = [
   "bsplus_user",
   "cloudAccessToken",
   "cloudUsername",
+  "bsplus_google_calendar",
 ] as const;
 
 /**
@@ -67,6 +68,7 @@ const AUTH_KEYS_TO_PRESERVE = [
   "bsplus_refresh_token",
   "bsplus_client_id",
   "bsplus_user",
+  "bsplus_google_calendar",
 ] as const;
 
 const OMIT_FROM_UPLOAD_EXACT = new Set<string>([