fix(storage): FoldersStorage.save is not transactional and load() never retries after a failure #1767

Problem

• `FoldersStorage.save` calls `LocalDb.clearFolders()` and then adds the folders one by one. A failure in the middle leaves only some of the folders.
• `FoldersStorage.load` sets `_loaded = true` even when reading failed and `_folders` is empty, so it never retries until `reload()`.

Fix

Replace the folder list in one transaction (a `replaceFolders` in `FolderRepository`), and only mark the storage loaded after a successful read.

Found in the system-functions review (see #1764).