modules/cleanup.nix: orphan cleanup on every switch

Every module imports cleanup.nix (also exported as
homeManagerModules.cleanup), which
- puts steam-frame-nix-cleanup (install.sh cleanup) on PATH;
- runs `cleanup --orphans --quiet` after linkGeneration on every switch,
  keeping what the configuration still uses (steamFrame.cleanup.keep, set
  by the modules: debugger while steamvrDebugger is on, the Firefox desktop
  profile's user.js while its fix applies); in a dry run it runs with
  --dry-run;
- runs `cleanup --all` instead when Home Manager's `uninstall = true;` is
  set (the manual uninstall route);
- removes links of older versions at paths Home Manager is about to own
  before checkLinkTargets (steamFrame.cleanup.migrateLinks), which would
  count as collisions otherwise.

install.sh cleanup gains --quiet (only actions, deferrals and warnings;
the header only when something is printed). The Jellyfin module's own
removal of the old shim copy goes: cleanup does it.

modules/firefox.nix

@@ -119,6 +119,8 @@ let
       --file-forwarding org.mozilla.firefox "''${profile[@]}" "$@"
   '');
 in {
+  imports = [ ./cleanup.nix ];
+
   options.steamFrame.firefox = {
     enable = lib.mkEnableOption ''
       the Firefox Flatpak (org.mozilla.firefox) launcher with Steam Frame
@@ -167,6 +169,8 @@ in {
       # Always, so disabling removes our user.js links (and old copies).
       home.activation.firefoxProfiles =
         lib.hm.dag.entryAfter [ "writeBoundary" ] "run ${profileSync}\n";
+      # The desktop profile's user.js link (steam-frame-nix-cleanup).
+      steamFrame.cleanup.keep = lib.optional desktopFix "firefox-desktop-userjs=${cfg.desktopProfile}";
     }
     (lib.mkIf cfg.enable {
       # stable: the branch the launcher runs (the extension point has no