feat: warn when Better Notes is not installed

Add isBetterNotesInstalled() helper and surface a warning notification
on startup plus clear errors during export stating Better Notes is a
requirement. Bump version to 1.0.3.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "zotero-org-export-annotations",
-  "version": "1.0.2",
+  "version": "1.0.3",
   "description": "Export Zotero PDF/EPUB annotations to Org-mode files for Emacs integration",
   "config": {
     "addonName": "Zotero Org Export Annotations",