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.

Fallback/fallback_test.sh

@@ -171,10 +171,6 @@ platform_require_cmd \
     "--version" "iptables" \
     "iptables" || { error "iptables not found — required for connectivity simulation"; exit 1; }
 
-platform_require_cmd \
-    "/usr/local/emhttp/plugins/dynamix/scripts/notify" \
-    "" "" \
-    "unRAID notify script" || warn "unRAID notify script not found — native notifications disabled"
 
 if [[ ! -f "$FALLBACK_SCRIPT" ]]; then
     error "fallback.sh not found at $FALLBACK_SCRIPT"