feat(games): add Snake and Tetris X11 games with menu integration

Add two standalone X11 games (not DWM plugins) for the Nokia 8110 keypad:
- src/games/snake.cpp: Nokia Snake II-style game, 240x296, built-in 5x7 font
- src/games/tetris.cpp: Tetris with next-piece preview, levels, line clears

Both float as dialog windows in dwm, use a self-contained bitmap font (no
font-file dependency), and are controlled via the numeric keypad (2/4/6/8
directions, 5 action) with arrow-key fallbacks.

- scripts/build-games.sh: Docker armv7 cross-compile (mirrors build-sxmo-x11.sh)
- overlay/rootfs/usr/share/sxmo/default_hooks/sxmo_hook_games.sh: Games menu hook
- overlay/rootfs/usr/bin/sxmo_appmenu.sh: add 'Games' entry after 'Apps' and a
  games submenu case

Binaries committed to prebuilt/games and overlay/rootfs/usr/bin per repo
convention. Deployed and verified on the live device (window maps, dwm focuses
it, keypad input reaches the game).

overlay/rootfs/usr/share/sxmo/default_hooks/sxmo_hook_games.sh

@@ -0,0 +1,26 @@
+#!/bin/sh
+# configversion: 00000000000000000000000000000000
+# SPDX-License-Identifier: AGPL-3.0-only
+# Copyright 2025 Bananarchy Contributors
+# shellcheck disable=SC2154
+
+# include common definitions
+# shellcheck source=configs/default_hooks/sxmo_hook_icons.sh
+export PATH="${HOME:-/home/user}/.local/bin:${HOME:-/home/user}/.config/sxmo/hooks:/usr/share/sxmo/default_hooks:$PATH"
+. sxmo_hook_icons.sh
+
+write_line() {
+	printf "%s ^ 0 ^ %s\n" "$1" "$2"
+}
+
+write_line_app() {
+	executable="$1"
+	label="$2"
+	command="$3"
+	if command -v "$executable" >/dev/null; then
+		write_line "$label" "$command"
+	fi
+}
+
+write_line_app snake "$icon_gam Snake" "snake"
+write_line_app tetris "$icon_gam Tetris" "tetris"