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/eventMap.ts

@@ -0,0 +1,58 @@
+import browser from "webextension-polyfill";
+
+export interface EventMapEntry {
+  id: string;
+  date: string;
+}
+
+export type EventMapRecord = Record<string, string | EventMapEntry>;
+
+export function eventMapKey(origin: string, seqtaKey: string): string {
+  return `${origin}::${seqtaKey}`;
+}
+
+export function normalizeEventMapEntry(
+  value: string | EventMapEntry | undefined,
+): EventMapEntry | undefined {
+  if (value == null) return undefined;
+  if (typeof value === "string") return { id: value, date: "" };
+  if (typeof value.id === "string" && value.id.length > 0) {
+    return { id: value.id, date: value.date ?? "" };
+  }
+  return undefined;
+}
+
+export function getStoredEventId(
+  value: string | EventMapEntry | undefined,
+): string | undefined {
+  return normalizeEventMapEntry(value)?.id;
+}
+
+export function lessonDateFromSeqtaKey(seqtaKey: string): string | undefined {
+  for (const part of seqtaKey.split(":")) {
+    if (/^\d{4}-\d{2}-\d{2}$/.test(part)) return part;
+  }
+  return undefined;
+}
+
+export function createCalendarStateStorage<T extends object>(storageKey: string) {
+  async function read(): Promise<T> {
+    const got = await browser.storage.local.get(storageKey);
+    const raw = got[storageKey];
+    if (!raw || typeof raw !== "object" || Array.isArray(raw)) return {} as T;
+    return raw as T;
+  }
+
+  async function write(patch: Partial<T>): Promise<T> {
+    const current = await read();
+    const next = { ...current, ...patch };
+    await browser.storage.local.set({ [storageKey]: next });
+    return next;
+  }
+
+  async function clear(): Promise<void> {
+    await browser.storage.local.remove(storageKey);
+  }
+
+  return { read, write, clear };
+}