[Feature Request] Include appmenu-gtk3-module in Flatpak build for KDE Global Menu support on Wayland

### Environment
- **OS:** Fedora Kinoite / Linux (KDE Plasma)
- **Session type:** Wayland
- **Package:** `io.gitlab.librewolf-community` (Flatpak)

### Problem Description
The KDE Plasma **Global Menu** widget does not display LibreWolf's menu bar when running LibreWolf under native Wayland in Flatpak, even when setting `widget.gtk.global-menu.enabled` to `true` in `about:config`.

Currently, the menu only exports successfully when forcing XWayland (`GDK_BACKEND=x11`).

### Root Cause
Under native Wayland, GTK applications require the `appmenu-gtk3-module` (or `dbusmenu-gtk3`) shared library to properly register window menus to the host's D-Bus interface (`com.canonical.AppMenu.Registrar` / `org.kde.kappmenu`).
Because the Flatpak bundle relies on the standard GNOME platform runtime, it lacks this specific module. By comparison, other Flatpaks like Betterbird (`eu.betterbird.Betterbird`) explicitly bundle `appmenu-gtk3-module` / `dbusmenu-gtk3` within their manifest, enabling native Wayland Global Menu integration out-of-the-box.

### Proposed Solution
Bundling `appmenu-gtk3-module` / `libdbusmenu-gtk3` (or including it in the Flatpak build manifest) so that LibreWolf can export its GTK menu bar to KDE Plasma under native Wayland without falling back to `GDK_BACKEND=x11`.

### Steps to Reproduce
1. Run LibreWolf Flatpak on a KDE Plasma Wayland session.
2. Set `widget.gtk.global-menu.enabled` to `true` in `about:config`.
3. Add the *Global Menu* widget to the KDE Plasma panel.
4. Notice that the menu bar disappears locally but does not show up in the KDE panel (unless `GDK_BACKEND=x11` override is applied).

Start by locating the Flatpak build manifest and reviewing how dependencies are bundled. Check whether appmenu-gtk3-module or libdbusmenu-gtk3 is available to the build, then reproduce the issue on KDE Plasma Wayland with the Global Menu widget. Done means the packaged application exports its menu without the GDK_BACKEND=x11 override.

Written by the indexing model from the issue text.