Platform-agnostic refactor: eliminate OS-specific hardcodes from core scripts

All bash scripts are now platform-neutral. Unraid-specific paths, commands,
and service checks moved to Plugin/unraid/adapter.sh. Core scripts call
platform_*() functions exclusively — no direct OS paths in runtime logic.

New adapter functions: platform_storage_path, platform_webui_install_path,
platform_scripts_dir_probe_cmd, platform_setup_db_path, platform_storage_healthy,
platform_is_service_enabled, platform_get_temp_thresholds, platform_disk_states_path,
platform_rebuild_container, platform_push_conf, platform_push_setup_state,
platform_get_templates_dir, platform_send_os_notification.

Partnership services stack (Emby/Jellyfin/Seerr/SeerrFin) added as third
onboarding stack alongside auth and arr stacks.

Docker_Essentials/docker_update.sh

@@ -336,7 +336,7 @@ if [[ ${#UPDATED[@]} -gt 0 ]]; then
             continue
         fi
         log "$ICON_SYNC Rebuilding $container from template on new image..."
-        if /usr/local/emhttp/plugins/dynamix.docker.manager/scripts/rebuild_container "$container" >/dev/null 2>&1; then
+        if platform_rebuild_container "$container"; then
             log "$ICON_DONE $container rebuilt ✅"
             REBUILT+=("$container")
         else