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/complete_uninstall.sh

@@ -7,7 +7,7 @@
 # (cups_do_uninstall, vnc_do_uninstall, wireguard_do_uninstall,
 # tailscale_do_uninstall, netbird_do_uninstall, lms_do_uninstall,
 # squeezelite_do_uninstall, asterisk_intercom_do_uninstall,
-# webui_do_uninstall, power_schedule_do_remove_all,
+# webui_do_uninstall, anki_sync_do_uninstall, power_schedule_do_remove_all,
 # emergency_hotspot_do_disable) instead of
 # re-implementing removal logic for each addon a second time here - if an
 # addon's uninstall logic changes, this picks it up automatically. Only
@@ -59,6 +59,7 @@ action_complete_uninstall() {
     echo "  • Remote access (VNC, WireGuard, Tailscale, Netbird)"
     echo "  • Asterisk Intercom (Baresip)"
     echo "  • Web UI"
+    echo "  • Anki Sync Server"
     echo "  • LightDM and Openbox"
     echo "  • All kiosk schedules and services"
     echo "  • Emergency hotspot configuration"
@@ -89,6 +90,7 @@ action_complete_uninstall() {
     squeezelite_do_uninstall
     asterisk_intercom_do_uninstall purge
     webui_do_uninstall
+    anki_sync_do_uninstall
 
     echo "[3/12] Removing schedules and emergency hotspot..."
     power_schedule_do_remove_all