Check for Update duplicated

Checking whether there is a new version available is done in two places:
- Client Dialogue
- Server Dialogue

with separate code to do the necessary network lookups.

1. There should only be one method implementing the check
2. The check should be unaware of whether it is running from a UI
3. Even when not running a UI, the user should be informed of a new version being available
4. When running a UI, they should be given the option to download and run the installer, exiting the program, if a new version is available

(Originally a comment in https://github.com/jamulussoftware/jamulus/pull/3556#discussion_r2724187266)

Start by locating the duplicated update checks in Client Dialogue and Server Dialogue, then compare their network lookup paths. Consolidate the check so it is independent of UI context, while preserving notification for non-UI use and the optional installer flow for UI use.

Written by the indexing model from the issue text.