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.

git_pull_execute.sh

@@ -255,7 +255,7 @@ else
     # ── Flash mode: sync Plugin/ to /boot/ so the webUI picks up updates ─────
     # In flash mode SCRIPTS_DIR is in appdata — Plugin/ lives in the repo there
     # but Unraid serves PHP from /boot/. Sync after every pull to keep them in step.
-    _BOOT_DIR="/boot/config/plugins/varaverk"
+    _BOOT_DIR=$(platform_webui_install_path)
     if [[ "$TARGET_DIR" != "$_BOOT_DIR" ]]; then
         echo ""
         echo "━━━ $ICON_SYNC Flash mode: sync Plugin/ → /boot/ ━━━"