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/clone_settings.sh

@@ -65,6 +65,7 @@ clone_detect_addons() {
     command -v netbird &>/dev/null && addons+=("netbird")
     baresip_is_installed 2>/dev/null && addons+=("asterisk_intercom")
     webui_is_installed 2>/dev/null && addons+=("webui")
+    anki_sync_is_installed 2>/dev/null && addons+=("anki_sync")
 
     if sudo -u "$KIOSK_USER" test -f "$CONFIG_PATH" 2>/dev/null; then
         local authelia_url
@@ -243,6 +244,13 @@ action_apply_clone_settings() {
                 fi ;;
             asterisk_intercom)
                 echo "  [!] Asterisk Intercom - needs its own SIP extension on this device" ;;
+            anki_sync)
+                if anki_sync_is_installed; then
+                    echo "  [x] Anki Sync Server - already installed"
+                else
+                    echo "  [ ] Anki Sync Server - not installed, install via Addons"
+                    missing_any=true
+                fi ;;
             authelia)
                 echo "  [!] Authelia - needs a fresh login on this device" ;;
             *)