feat: add S3-compatible storage provider (MinIO, Ceph, R2, etc.)

Adds a new 'S3-Compatible Storage' provider that works with any
S3-API-compatible object storage service, including MinIO, Ceph,
Cloudflare R2, Backblaze B2, and others.

Changes:
- New provider class: classes/providers/storage/s3-compatible-provider.php
  - Provider key: s3compatible
  - Reads user-configured endpoint URL from settings
  - Uses path-style URL access (required by most S3-compatible services)
  - Supports credentials via AS3CF_S3COMPAT_ACCESS_KEY_ID /
    AS3CF_S3COMPAT_SECRET_ACCESS_KEY wp-config.php constants
  - Disables AWS-specific features (Block Public Access, Object Ownership)
- New provider SVG icons (s3compatible.svg, -link.svg, -round.svg)
- Registered provider in main plugin class with endpoint setting support
- Updated StorageProviderSubPage to show endpoint URL input for S3-compatible
- Built pro settings bundle with rollup (Svelte 4.2.19)
- Added package.json and updated rollup.config.mjs for pro-only builds

classes/pro/integrations/assets/api/v1/assets-domain-check.php

@@ -0,0 +1,59 @@
+<?php
+
+namespace DeliciousBrains\WP_Offload_Media\Pro\Integrations\Assets\API\V1;
+
+use DeliciousBrains\WP_Offload_Media\Pro\API\API;
+use DeliciousBrains\WP_Offload_Media\Pro\Integrations\Assets\Domain_Check_Response;
+use WP_REST_Request;
+use WP_REST_Response;
+
+class Assets_Domain_Check extends API {
+	/** @var int */
+	protected static $version = 1;
+
+	/** @var string */
+	protected static $name = 'assets-domain-check';
+
+	/**
+	 * Register REST API routes.
+	 */
+	public function register_routes() {
+		register_rest_route(
+			static::api_namespace(),
+			static::route() . '(?P<key>[\w\d=]+)',
+			array(
+				'methods'             => 'GET',
+				'callback'            => array( $this, 'get_domain_check' ),
+				'permission_callback' => '__return_true', // public access
+			)
+		);
+	}
+
+	/**
+	 * Respond to a GET request to the domain check route, with the given key.
+	 *
+	 * @param WP_REST_Request $request
+	 *
+	 * @return WP_REST_Response
+	 */
+	public function get_domain_check( WP_REST_Request $request ): WP_REST_Response {
+		$response = new Domain_Check_Response( array(
+			'key' => $request->get_param( 'key' ),
+			'ver' => filter_input( INPUT_GET, 'ver' ), // must come in as url param
+		) );
+		$response->header( 'X-As3cf-Signature', $response->hashed_signature() );
+
+		return $this->rest_ensure_response( 'get', static::name(), $response );
+	}
+
+	/**
+	 * Get a URL to the domain check route, with the given key.
+	 *
+	 * @param string $key
+	 *
+	 * @return string
+	 */
+	public function get_url( string $key ): string {
+		return rest_url( static::endpoint() . $key );
+	}
+}