Add `home_system_restart_required` flag

A single flag saying whether a home system restart is pending, one for the whole system rather than one per source. Anything that needs one can set it, including a finished update, and identifies itself so the frontend can show what asked. It stays set until the restart happens.

Start by searching the repository for existing per-source restart-required handling and the frontend path that displays restart requests. Define how one system-wide pending flag records its requester, how completed updates can set it, and how the restart clears it; done means the frontend can show the requester and the flag remains set until restart.

Written by the indexing model from the issue text.