Route the Background portal to xdg-desktop-portal-gnome

Nothing served org.freedesktop.impl.portal.Background under Hyprland:
gnome.portal implements it but is UseIn=gnome, same situation the
Secret mapping below already solves for gnome-keyring. Every Flatpak
app's own "run/start on login" toggle goes through this portal, so it
silently failed for all of them -- Bitwarden logged the failure on
every launch.

Verified live rather than assumed: xdg-desktop-portal-gnome starts as
a standalone backend process with no dependency on GNOME Shell or
gnome-session, and after adding the mapping and restarting the portal,
Bitwarden's own autostart request succeeded end to end
(response=Background { background: true, autostart: true }) where it
previously errored with "portal frontend ... was not found".

Claude-Session: https://claude.ai/code/session_01E6TJUAh41HaP25MVHWkhRZ

config/dot/xdg-desktop-portal/hyprland-portals.conf

@@ -15,9 +15,18 @@
 #   Secret    -> gnome-keyring. Its own portal file is UseIn=gnome, so under
 #                XDG_CURRENT_DESKTOP=Hyprland nothing would claim Secret and
 #                Flatpak apps needing credential storage would fail.
+#   Background -> xdg-desktop-portal-gnome. It's the only installed backend
+#                that implements this interface (gnome.portal is UseIn=gnome,
+#                same problem as Secret above), and it runs as a standalone
+#                daemon that doesn't need GNOME Shell or gnome-session --
+#                confirmed live: it starts cleanly under Hyprland and Flatpak
+#                apps' RequestBackground calls succeed. Without this mapping
+#                nothing serves org.freedesktop.portal.Background at all, so
+#                any Flatpak app's own "run/start on login" toggle silently
+#                fails (Bitwarden logged this on every launch).
 #
-# xdg-desktop-portal-gnome stays installed but is inert here (its portal file
-# is UseIn=gnome); it is still needed for the GNOME session.
+# xdg-desktop-portal-gnome stays installed and is now used for one interface
+# here; it is still the default handler for the GNOME session.
 # ─────────────────────────────────────────────────────────────────────────────
 
 [preferred]
@@ -25,3 +34,4 @@ default=hyprland;gtk
 org.freedesktop.impl.portal.Screenshot=hyprland
 org.freedesktop.impl.portal.ScreenCast=hyprland
 org.freedesktop.impl.portal.Secret=gnome-keyring
+org.freedesktop.impl.portal.Background=gnome