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

lib/config.sh

@@ -33,6 +33,8 @@
 : "${WIREGUARD_DIR:=/etc/wireguard}"
 : "${WEBUI_DIR:=/opt/kiosk-webui}"
 : "${WEBUI_ENV_DIR:=/etc/kiosk-webui}"
+: "${ANKI_SYNC_DIR:=/opt/kiosk-anki-sync}"
+: "${ANKI_SYNC_ENV_DIR:=/etc/kiosk-anki-sync}"
 : "${SUDOERS_D_DIR:=/etc/sudoers.d}"
 # The one root-owned script the Web UI's addon-install/Update actions are
 # allowed to invoke via passwordless sudo (menus/addon_webui.sh writes it