refactor: consolidate and debloat calendar sync implementation

Merge duplicated Google/Outlook engines, background handlers, and API
layers into shared calendarSync modules to cut complexity without changing
user-facing sync behavior.

src/seqta/utils/calendarSync/settings.ts

@@ -1,20 +1,53 @@
+import browser from "webextension-polyfill";
 import {
-  GOOGLE_CALENDAR_SYNC_WEEKS_DEFAULT,
+  GOOGLE_CALENDAR_SYNC_WEEKS,
   GOOGLE_CALENDAR_SYNC_WEEKS_MAX,
   GOOGLE_CALENDAR_SYNC_WEEKS_MIN,
 } from "@/config/googleCalendar";
-import { readOutlookCalendarState } from "@/seqta/utils/outlookCalendar/storage";
 import { readGoogleCalendarState } from "@/seqta/utils/googleCalendar/storage";
-import {
-  readSharedCalendarSyncSettings,
-  WEEKLY_SYNC_INTERVAL_MS,
-  writeSharedCalendarSyncSettings,
-} from "./sharedSettings";
+import { readOutlookCalendarState } from "@/seqta/utils/outlookCalendar/storage";
 
-export { CALENDAR_WEEKLY_ALARM, WEEKLY_SYNC_INTERVAL_MS } from "./sharedSettings";
+export const BSPLUS_CALENDAR_SYNC_SETTINGS_KEY = "bsplus_calendar_sync_settings";
+export const CALENDAR_WEEKLY_ALARM = "bsplus_calendar_weekly";
+export const WEEKLY_SYNC_INTERVAL_MS = 7 * 24 * 60 * 60 * 1000;
+
+export interface SharedCalendarSyncSettings {
+  syncWeeksAhead?: number;
+  autoSyncWeekly?: boolean;
+  lastWeeklySyncAt?: number;
+  pendingWeeklySync?: boolean;
+}
+
+export async function readSharedCalendarSyncSettings(): Promise<SharedCalendarSyncSettings> {
+  const got = await browser.storage.local.get(BSPLUS_CALENDAR_SYNC_SETTINGS_KEY);
+  const raw = got[BSPLUS_CALENDAR_SYNC_SETTINGS_KEY];
+  const shared =
+    raw && typeof raw === "object" && !Array.isArray(raw)
+      ? (raw as SharedCalendarSyncSettings)
+      : {};
+
+  if (Object.keys(shared).length > 0) return shared;
+
+  const legacy = await readGoogleCalendarState();
+  return {
+    syncWeeksAhead: legacy.syncWeeksAhead,
+    autoSyncWeekly: legacy.autoSyncWeekly,
+    lastWeeklySyncAt: legacy.lastWeeklySyncAt,
+    pendingWeeklySync: legacy.pendingWeeklySync,
+  };
+}
+
+export async function writeSharedCalendarSyncSettings(
+  patch: Partial<SharedCalendarSyncSettings>,
+): Promise<SharedCalendarSyncSettings> {
+  const current = await readSharedCalendarSyncSettings();
+  const next = { ...current, ...patch };
+  await browser.storage.local.set({ [BSPLUS_CALENDAR_SYNC_SETTINGS_KEY]: next });
+  return next;
+}
 
 export function clampSyncWeeks(weeks: number): number {
-  if (!Number.isFinite(weeks)) return GOOGLE_CALENDAR_SYNC_WEEKS_DEFAULT;
+  if (!Number.isFinite(weeks)) return GOOGLE_CALENDAR_SYNC_WEEKS;
   return Math.min(
     GOOGLE_CALENDAR_SYNC_WEEKS_MAX,
     Math.max(GOOGLE_CALENDAR_SYNC_WEEKS_MIN, Math.round(weeks)),
@@ -23,7 +56,7 @@ export function clampSyncWeeks(weeks: number): number {
 
 export async function getSyncWeeksAhead(): Promise<number> {
   const settings = await readSharedCalendarSyncSettings();
-  return clampSyncWeeks(settings.syncWeeksAhead ?? GOOGLE_CALENDAR_SYNC_WEEKS_DEFAULT);
+  return clampSyncWeeks(settings.syncWeeksAhead ?? GOOGLE_CALENDAR_SYNC_WEEKS);
 }
 
 export async function getAutoSyncWeekly(): Promise<boolean> {
@@ -31,7 +64,7 @@ export async function getAutoSyncWeekly(): Promise<boolean> {
   return settings.autoSyncWeekly !== false;
 }
 
-async function isAnyCalendarConnected(): Promise<boolean> {
+export async function isAnyCalendarConnected(): Promise<boolean> {
   const [google, outlook] = await Promise.all([
     readGoogleCalendarState(),
     readOutlookCalendarState(),