flatpak: Add --device=all permission

Without --device=all, the sandbox gets a dedicated PTY namespace.
Commands run on the host via the HostCommand D-Bus interface receive the
file descriptors from the namespaced PTY but cannot determine its path
via ttyname(3). This breaks commands like tty(1), ps(1) and
emacsclient(1).

Add --device=all so the host PTY namespace is used when allocating TTYs.
Applications with access to org.freedesktop.Flatpak can already give
themselves arbitrary permissions, so the sandboxing benefits of
restricted device access are limited. For terminal emulators, the
primary benefit of Flatpak is the predictability of the
distro-independent target runtime rather than sandboxing.

flatpak/com.mitchellh.ghostty.Devel.yml

@@ -14,6 +14,8 @@ desktop-file-name-suffix: " (Devel)"
 finish-args:
   # 3D rendering
   - --device=dri
+  # use host PTS namespace
+  - --device=all
   # Windowing
   - --share=ipc
   - --socket=fallback-x11

flatpak/com.mitchellh.ghostty.yml

@@ -9,6 +9,8 @@ command: ghostty
 finish-args:
   # 3D rendering
   - --device=dri
+  # use host PTS namespace
+  - --device=all
   # Windowing
   - --share=ipc
   - --socket=fallback-x11