Non-blocking title and description fetching

The app must load missing title or description in background. The link without title or description still should be added into the list, but spinning circles should be used to denote missing fields.

About storage, if the user provided:
* no title and no description — we store `.link` file, but we don't store anything in `user/properties`
* only description — besides `.link` file, we store JSON with single field `description`
* only title — same, we create JSON with single `title` field
* both — we create JSON with both

The `cache/metadata` storage should be used to store automatically fetched title and description, this is as fallback for `user/properties` (when 1 or 2 fields are missing). If a title or description is missing in both properties and cache, then we fetch them from the web. If there is no internet connectivity, then spinning circle can be replaced by dedicated "offline" icon.

Start by tracing how links are added to the list and how user/properties, .link files, and cache/metadata are read. Verify the cases where title or description is missing, then check that background fetching, spinning indicators, and the offline icon produce the requested fallback behavior.

Written by the indexing model from the issue text.