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/syncRunner.ts

@@ -0,0 +1,86 @@
+import browser from "webextension-polyfill";
+import { fetchTimetableForSync, fetchTimetableLessons } from "@/seqta/utils/googleCalendar/fetchTimetable";
+import { trailingWeekRange } from "@/seqta/utils/googleCalendar/syncDateRange";
+import { getSyncWeeksAhead } from "@/seqta/utils/calendarSync/settings";
+import { reportSyncProgress } from "@/seqta/utils/calendarSync/lessonSyncShared";
+import {
+  googleLessonSyncProvider,
+  outlookLessonSyncProvider,
+  syncLessonsToCalendar,
+  type CalendarLessonSyncProvider,
+} from "@/seqta/utils/calendarSync/syncEngine";
+import type {
+  GoogleCalendarSyncProgress,
+  GoogleCalendarSyncResult,
+} from "@/seqta/utils/googleCalendar/types";
+
+export interface RunCalendarSyncParams {
+  mode?: "full" | "incremental";
+  onProgress?: (progress: GoogleCalendarSyncProgress) => void;
+}
+
+type CalendarSyncRunnerConfig = {
+  accessTokenMessageType: string;
+  accessTokenError: string;
+  lessonSyncProvider: CalendarLessonSyncProvider;
+};
+
+const GOOGLE_SYNC_RUNNER: CalendarSyncRunnerConfig = {
+  accessTokenMessageType: "googleCalendarGetAccessToken",
+  accessTokenError: "Could not get Google Calendar access token.",
+  lessonSyncProvider: googleLessonSyncProvider,
+};
+
+const OUTLOOK_SYNC_RUNNER: CalendarSyncRunnerConfig = {
+  accessTokenMessageType: "outlookCalendarGetAccessToken",
+  accessTokenError: "Could not get Outlook Calendar access token.",
+  lessonSyncProvider: outlookLessonSyncProvider,
+};
+
+async function getAccessTokenFromBackground(
+  messageType: string,
+  errorMessage: string,
+): Promise<string> {
+  const res = (await browser.runtime.sendMessage({ type: messageType })) as {
+    success?: boolean;
+    accessToken?: string;
+    error?: string;
+  };
+  if (!res?.success || !res.accessToken) {
+    throw new Error(res?.error ?? errorMessage);
+  }
+  return res.accessToken;
+}
+
+export async function runCalendarSync(
+  config: CalendarSyncRunnerConfig,
+  params: RunCalendarSyncParams = {},
+): Promise<GoogleCalendarSyncResult> {
+  const mode = params.mode ?? "full";
+  const weeksAhead = await getSyncWeeksAhead();
+
+  reportSyncProgress(params.onProgress, {
+    phase: "preparing",
+    current: 0,
+    total: 1,
+    message: mode === "incremental" ? "Fetching new week…" : "Fetching timetable…",
+  });
+
+  const lessons =
+    mode === "incremental"
+      ? await fetchTimetableLessons(trailingWeekRange(weeksAhead))
+      : await fetchTimetableForSync(weeksAhead);
+
+  return syncLessonsToCalendar(
+    config.lessonSyncProvider,
+    { origin: location.origin, lessons, mode, weeksAhead },
+    () => getAccessTokenFromBackground(config.accessTokenMessageType, config.accessTokenError),
+    { onProgress: params.onProgress },
+  );
+}
+
+export const runGoogleCalendarSync = (params?: RunCalendarSyncParams) =>
+  runCalendarSync(GOOGLE_SYNC_RUNNER, params);
+
+export const runOutlookCalendarSync = (params?: RunCalendarSyncParams) =>
+  runCalendarSync(OUTLOOK_SYNC_RUNNER, params);