A new platform feature is defined, `@akkoma/local-only`, which is marked as present only if the string `akkoma` is found in the instance version string.
If this feature is supported, a new post visibility option is added to the post compose visibility selector, "Local instance", which sets the visibility to `"local"`.
The post viewer is also updated accordingly; while local-only posts could already be seen previously, they didn't have anything written in the `.extra-meta` section, and didn't display a visibility icon.
The `building` icon has been selected as the symbol for "Local instance" posts, as it's already used in the main menu to denote the Local timeline.
