Add storage permission for account persistence

Include the Thunderbird storage permission required for browser.storage.local and make AccountStore fail fast when storage.local is unavailable.

manifest.json

@@ -10,7 +10,8 @@
     }
   },
   "permissions": [
-    "cloudFile"
+    "cloudFile",
+    "storage"
   ],
   "host_permissions": [
     "http://*/*",

src/cloudfile/account-store.js

@@ -1,8 +1,19 @@
 const ACCOUNT_PREFIX = "account:";
 const FILE_PREFIX = "uploaded-file:";
 
+function resolveStorageArea() {
+  const extensionApi = globalThis.browser ?? globalThis.messenger;
+  const storageArea = extensionApi?.storage?.local;
+
+  if (!storageArea) {
+    throw new Error("Extension storage.local is unavailable. Check the manifest permissions for the add-on.");
+  }
+
+  return storageArea;
+}
+
 export class AccountStore {
-  constructor(storageArea = globalThis.browser?.storage?.local) {
+  constructor(storageArea = resolveStorageArea()) {
     this.storageArea = storageArea;
   }