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/Signature/SignatureTrait.php

@@ -0,0 +1,34 @@
+<?php
+
+namespace DeliciousBrains\WP_Offload_Media\Aws3\Aws\Signature;
+
+/**
+ * Provides signature calculation for SignatureV4.
+ */
+trait SignatureTrait
+{
+    /** @var array Cache of previously signed values */
+    private $cache = [];
+    /** @var int Size of the hash cache */
+    private $cacheSize = 0;
+    private function createScope($shortDate, $region, $service)
+    {
+        return "{$shortDate}/{$region}/{$service}/aws4_request";
+    }
+    private function getSigningKey($shortDate, $region, $service, $secretKey)
+    {
+        $k = $shortDate . '_' . $region . '_' . $service . '_' . $secretKey;
+        if (!isset($this->cache[$k])) {
+            // Clear the cache when it reaches 50 entries
+            if (++$this->cacheSize > 50) {
+                $this->cache = [];
+                $this->cacheSize = 0;
+            }
+            $dateKey = \hash_hmac('sha256', $shortDate, "AWS4{$secretKey}", \true);
+            $regionKey = \hash_hmac('sha256', $region, $dateKey, \true);
+            $serviceKey = \hash_hmac('sha256', $service, $regionKey, \true);
+            $this->cache[$k] = \hash_hmac('sha256', 'aws4_request', $serviceKey, \true);
+        }
+        return $this->cache[$k];
+    }
+}