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/Identity/S3/S3ExpressIdentityProvider.php

@@ -0,0 +1,41 @@
+<?php
+
+namespace DeliciousBrains\WP_Offload_Media\Aws3\Aws\Identity\S3;
+
+use DeliciousBrains\WP_Offload_Media\Aws3\Aws;
+use DeliciousBrains\WP_Offload_Media\Aws3\Aws\LruArrayCache;
+use DeliciousBrains\WP_Offload_Media\Aws3\GuzzleHttp\Promise;
+class S3ExpressIdentityProvider
+{
+    private $cache;
+    private $region;
+    private $config;
+    private $s3Client;
+    public function __construct($clientRegion, array $config = [])
+    {
+        $this->cache = new LruArrayCache(100);
+        $this->region = $clientRegion;
+        $this->config = $config;
+    }
+    public function __invoke($command)
+    {
+        $s3Client = $this->getS3Client();
+        $bucket = $command['Bucket'];
+        if ($identity = $this->cache->get($bucket)) {
+            if (!$identity->isExpired()) {
+                return Promise\Create::promiseFor($identity);
+            }
+        }
+        $response = $s3Client->createSession(['Bucket' => $bucket]);
+        $identity = new Aws\Identity\S3\S3ExpressIdentity($response['Credentials']['AccessKeyId'], $response['Credentials']['SecretAccessKey'], $response['Credentials']['SessionToken'], $response['Credentials']['Expiration']->getTimestamp());
+        $this->cache->set($bucket, $identity);
+        return Promise\Create::promiseFor($identity);
+    }
+    private function getS3Client()
+    {
+        if (\is_null($this->s3Client)) {
+            $this->s3Client = $this->config['client'] ?? new Aws\S3\S3Client(['region' => $this->region, 'disable_express_session_auth' => \true]);
+        }
+        return $this->s3Client;
+    }
+}