Add Anki Sync Server addon (self-hosted flashcard sync)

Installs the official self-hosted anki-sync-server (python -m
anki.syncserver from PyPI's anki package, no Qt/GUI deps) as a
systemd service under $KIOSK_USER, same shape as the Web UI addon:
venv under $ANKI_SYNC_DIR, EnvironmentFile under $ANKI_SYNC_ENV_DIR
(chmod 600 since it holds plaintext sync passwords), multi-user
support via SYNC_USER1/2/... Lets Anki desktop/AnkiDroid clients on
the LAN sync flashcards against this kiosk instead of AnkiWeb.

Added to the existing Addons menu rather than a new top-level
category - Addons is already a flat mix of unrelated self-hosted
services (printing, telephony, media server, remote access), so one
more service fits that pattern instead of fragmenting the menu
structure for a single item.

Wired into complete_uninstall.sh, clone_settings.sh's addon
detection/checklist, and the webui helper script's source list (which
documents itself as mirroring install.sh's sources exactly).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NoqDrsMq2dazAwGU1AodNc

menus/addon_webui.sh

@@ -172,6 +172,7 @@ source "$SCRIPT_DIR/menus/addon_remote_access.sh"
 source "$SCRIPT_DIR/menus/addon_lms_squeezelite.sh"
 source "$SCRIPT_DIR/menus/addon_asterisk_intercom.sh"
 source "$SCRIPT_DIR/menus/addon_webui.sh"
+source "$SCRIPT_DIR/menus/addon_anki_sync.sh"
 source "$SCRIPT_DIR/menus/advanced_electron.sh"
 source "$SCRIPT_DIR/menus/advanced_upgrade.sh"
 source "$SCRIPT_DIR/menus/advanced_factory_reset.sh"