Give libadwaita applications the desktop's accent

Files, Papers, Loupe and every other libadwaita application read their
accent from the Settings portal, so they rendered in GNOME blue no
matter which accent this desktop was set to -- correct on our own
surfaces, wrong on half the screen.

xdg-desktop-portal-gtk cannot serve org.freedesktop.appearance
accent-color at all; the string does not appear in the 1.15.3 binary.
The gnome backend serves it, so Settings now routes to gnome with gtk
still listed behind it -- the frontend merges Settings backends in
order, so color-scheme keeps resolving if the gnome backend is ever
unavailable.

GNOME's accent-color is a fixed enum of nine names rather than a color,
so each of our eight accents carries its nearest member. Nearest by hue
rather than by name: rose maps to red, because it is the red role in
this palette.

Claude-Session: https://claude.ai/code/session_01BRvzt4H8XXLPVH5MyYdk9L

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

@@ -7,7 +7,8 @@
 # Why each line:
 #   hyprland  -> Screenshot, ScreenCast, GlobalShortcuts, InputCapture
 #                (this is what makes OBS / Sunshine / Zoom screen sharing work)
-#   gtk       -> FileChooser, Settings, Notification, Print, Access
+#   gtk       -> FileChooser, Notification, Print, Access, and Settings as
+#                the fallback behind gnome (see below)
 #                Settings is load-bearing: xdg-desktop-portal-gtk reads
 #                gsettings' org.gnome.desktop.interface and serves
 #                org.freedesktop.appearance, which is how libadwaita apps know
@@ -25,7 +26,17 @@
 #                any Flatpak app's own "run/start on login" toggle silently
 #                fails (Bitwarden logged this on every launch).
 #
-# xdg-desktop-portal-gnome stays installed and is now used for one interface
+#   Settings  -> gnome FIRST, gtk behind it. The frontend merges Settings
+#                backends in order, so this is additive rather than a swap.
+#                It has to be this way round because xdg-desktop-portal-gtk
+#                cannot serve org.freedesktop.appearance accent-color at all --
+#                the string does not appear in the 1.15.3 binary -- so
+#                libadwaita applications rendered in GNOME blue no matter what
+#                accent this desktop was set to. The gnome backend serves it,
+#                and gtk staying listed means color-scheme still resolves if
+#                the gnome backend is ever unavailable.
+#
+# xdg-desktop-portal-gnome stays installed and is now used for two interfaces
 # here; it is still the default handler for the GNOME session.
 # ─────────────────────────────────────────────────────────────────────────────
 
@@ -35,3 +46,4 @@ 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
+org.freedesktop.impl.portal.Settings=gnome;gtk