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

vendor/Aws3/Aws/Api/Serializer/RestJsonSerializer.php

@@ -0,0 +1,34 @@
+<?php
+
+namespace DeliciousBrains\WP_Offload_Media\Aws3\Aws\Api\Serializer;
+
+use DeliciousBrains\WP_Offload_Media\Aws3\Aws\Api\Service;
+use DeliciousBrains\WP_Offload_Media\Aws3\Aws\Api\StructureShape;
+/**
+ * Serializes requests for the REST-JSON protocol.
+ * @internal
+ */
+class RestJsonSerializer extends RestSerializer
+{
+    /** @var JsonBody */
+    private $jsonFormatter;
+    /** @var string */
+    private $contentType;
+    /**
+     * @param Service  $api           Service API description
+     * @param string   $endpoint      Endpoint to connect to
+     * @param JsonBody $jsonFormatter Optional JSON formatter to use
+     */
+    public function __construct(Service $api, $endpoint, JsonBody $jsonFormatter = null)
+    {
+        parent::__construct($api, $endpoint);
+        $this->contentType = JsonBody::getContentType($api);
+        $this->jsonFormatter = $jsonFormatter ?: new JsonBody($api);
+    }
+    protected function payload(StructureShape $member, array $value, array &$opts)
+    {
+        $body = isset($value) ? (string) $this->jsonFormatter->build($member, $value) : "{}";
+        $opts['headers']['Content-Type'] = $this->contentType;
+        $opts['body'] = $body;
+    }
+}