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/lib/extensionAssetUrl.ts

@@ -1,18 +1,9 @@
 import browser from "webextension-polyfill";
 
-/** Vite asset imports are often already absolute extension URLs in production bundles. */
-export function resolveExtensionAssetUrl(importedUrl: string): string {
-  if (!importedUrl) return importedUrl;
-
-  const doublePrefix = importedUrl.match(
-    /^((?:chrome|moz)-extension:\/\/[^/]+)\/\1\/(.+)$/,
-  );
-  if (doublePrefix) {
-    return `${doublePrefix[1]}/${doublePrefix[2]}`;
-  }
-
-  if (/^(chrome-extension|moz-extension|https?|data):/.test(importedUrl)) {
-    return importedUrl;
-  }
-  return browser.runtime.getURL(importedUrl.replace(/^\/+/, ""));
+/** Resolve Vite asset imports to a usable extension URL. */
+export function resolveExtensionAssetUrl(url: string): string {
+  const doubled = url.match(/^((?:chrome|moz)-extension:\/\/[^/]+)\/\1\/(.+)$/);
+  if (doubled) return `${doubled[1]}/${doubled[2]}`;
+  if (/^(?:chrome|moz)-extension:\/\/|https?:|data:/.test(url)) return url;
+  return browser.runtime.getURL(url.replace(/^\/+/, ""));
 }