Add run-on-frame.sh to launch apps on the headset desktop

Borrows the nested Plasma session's display and D-Bus variables from
plasmashell and starts the app detached. Tested on the Frame: error paths,
argument quoting, ~ expansion, and `mac-screen` opening a VNC connection.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

scripts/run-on-frame.sh

@@ -0,0 +1,75 @@
+#!/usr/bin/env zsh
+# Mac-side: start a GUI app on the Steam Frame's in-headset desktop over SSH.
+#
+# The headset desktop is a nested Plasma session inside gamescope with its own
+# runtime dir, Wayland socket, X display and D-Bus bus (verified 2026-09-25), so
+# a plain `ssh frame some-app` can't find it. This copies those variables from
+# plasmashell's environment, then starts the app detached so it outlives SSH.
+#
+# Usage:
+#   scripts/run-on-frame.sh remmina              # Remmina main window
+#   scripts/run-on-frame.sh mac-screen           # Remmina straight into the Mac profile
+#   scripts/run-on-frame.sh org.example.App [ARGS...]  # any installed Flatpak
+#   scripts/run-on-frame.sh -- COMMAND [ARGS...]       # any command on the Frame
+#
+# A leading "~/" in any argument is expanded on the Frame.
+set -euo pipefail
+
+FRAME_ALIAS=${FRAME_ALIAS:-frame}
+REMMINA_PROFILE="~/.var/app/org.remmina.Remmina/data/remmina/mac-screen-sharing.remmina"
+
+case "${1:-}" in
+  ""|-h|--help) sed -n '2,15p' "$0"; exit 0 ;;
+  remmina)      cmd=(flatpak run org.remmina.Remmina) ;;
+  mac-screen)   cmd=(flatpak run org.remmina.Remmina -c "$REMMINA_PROFILE") ;;
+  --)           shift; (( $# )) || { print -u2 "run-on-frame: missing command after --"; exit 2; }
+                cmd=("$@") ;;
+  *)
+    if [[ "$1" =~ '^[A-Za-z0-9_-]+(\.[A-Za-z0-9_-]+){2,}$' ]]; then
+      cmd=(flatpak run "$@")
+    else
+      print -u2 "run-on-frame: unknown app '$1' (use a Flatpak app ID, or -- COMMAND)"
+      exit 2
+    fi ;;
+esac
+
+# Runs on the Frame with the command as "$@".
+remote=$(cat <<'EOF'
+set -u
+pid=$(pgrep -u "$(id -u)" -x plasmashell | head -n 1)
+if [ -z "$pid" ]; then
+  echo "plasmashell is not running: open the desktop in the headset first." >&2
+  exit 2
+fi
+while IFS= read -r -d '' kv; do
+  case "$kv" in
+    WAYLAND_DISPLAY=*|XDG_RUNTIME_DIR=*|DISPLAY=*|XAUTHORITY=*|DBUS_SESSION_BUS_ADDRESS=*|\
+    XDG_DATA_DIRS=*|XDG_CURRENT_DESKTOP=*|XDG_SESSION_TYPE=*) export "$kv" ;;
+  esac
+done < "/proc/$pid/environ"
+args=()
+for a in "$@"; do
+  case "$a" in "~/"*) a="$HOME/${a#\~/}" ;; esac
+  args+=("$a")
+done
+log=$(mktemp /tmp/run-on-frame.XXXXXX)
+setsid nohup "${args[@]}" > "$log" 2>&1 < /dev/null &
+child=$!
+sleep 3
+if kill -0 "$child" 2>/dev/null; then
+  echo "started: ${args[*]} (pid $child, log $log)"
+elif wait "$child"; then
+  # Single-instance apps (e.g. Remmina) hand off to the running copy and exit 0.
+  echo "done: ${args[*]} (exited 0; single-instance apps hand off to the running copy)"
+  rm -f "$log"
+else
+  rc=$?
+  echo "failed: ${args[*]} (exit $rc). Log:" >&2
+  tail -n 20 "$log" >&2
+  exit 1
+fi
+EOF
+)
+b64=$(print -rn -- "$remote" | base64)
+
+ssh "$FRAME_ALIAS" "bash -c \"\$(echo $b64 | base64 -d)\" run-on-frame ${(j: :)${(@q)cmd}}"