feat: "Start Steam silently in desktop mode" is opt-in (it was always on); an existing systemd unit counts as on and is migrated by the update

TECHNICAL.md

@@ -109,7 +109,7 @@ tools write to the conf.d fragment into the base config.
   installs `/usr/local/bin/sync-steamos-session.sh` plus
   `sync-steamos-session.path`/`.service`;
 - `KWIN_IM_SHOW_ALWAYS=1` for the virtual keyboard; the sub-option `silent`
-  ("Start Steam silently in desktop mode", on by default, always shown) writes
+  ("Start Steam silently in desktop mode", opt-in, always shown) writes
   `~/.config/autostart/steam.desktop` (the system steam.desktop with `-silent` in its Exec line). That is the
   file Steam's own setting "Run Steam when my computer starts" creates and removes, so the setting shows it. An older
   `steam-desktop-autostart` user unit is removed;