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-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "zotero-org-export-annotations",
-  "version": "1.0.2",
+  "version": "1.0.3",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "zotero-org-export-annotations",
-      "version": "1.0.2",
+      "version": "1.0.3",
       "license": "GPL-3.0-or-later",
       "devDependencies": {
         "@types/node": "^20.11.0",