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/S3/SSECMiddleware.php

@@ -0,0 +1,62 @@
+<?php
+
+namespace DeliciousBrains\WP_Offload_Media\Aws3\Aws\S3;
+
+use DeliciousBrains\WP_Offload_Media\Aws3\Aws\CommandInterface;
+use DeliciousBrains\WP_Offload_Media\Aws3\Psr\Http\Message\RequestInterface;
+/**
+ * Simplifies the SSE-C process by encoding and hashing the key.
+ * @internal
+ */
+class SSECMiddleware
+{
+    private $endpointScheme;
+    private $nextHandler;
+    /**
+     * Provide the URI scheme of the client sending requests.
+     *
+     * @param string $endpointScheme URI scheme (http/https).
+     *
+     * @return callable
+     */
+    public static function wrap($endpointScheme)
+    {
+        return function (callable $handler) use($endpointScheme) {
+            return new self($endpointScheme, $handler);
+        };
+    }
+    public function __construct($endpointScheme, callable $nextHandler)
+    {
+        $this->nextHandler = $nextHandler;
+        $this->endpointScheme = $endpointScheme;
+    }
+    public function __invoke(CommandInterface $command, RequestInterface $request = null)
+    {
+        // Allows only HTTPS connections when using SSE-C
+        if (($command['SSECustomerKey'] || $command['CopySourceSSECustomerKey']) && $this->endpointScheme !== 'https') {
+            throw new \RuntimeException('You must configure your S3 client to ' . 'use HTTPS in order to use the SSE-C features.');
+        }
+        // Prepare the normal SSE-CPK headers
+        if ($command['SSECustomerKey']) {
+            $this->prepareSseParams($command);
+        }
+        // If it's a copy operation, prepare the SSE-CPK headers for the source.
+        if ($command['CopySourceSSECustomerKey']) {
+            $this->prepareSseParams($command, 'CopySource');
+        }
+        $f = $this->nextHandler;
+        return $f($command, $request);
+    }
+    private function prepareSseParams(CommandInterface $command, $prefix = '')
+    {
+        // Base64 encode the provided key
+        $key = $command[$prefix . 'SSECustomerKey'];
+        $command[$prefix . 'SSECustomerKey'] = \base64_encode($key);
+        // Base64 the provided MD5 or, generate an MD5 if not provided
+        if ($md5 = $command[$prefix . 'SSECustomerKeyMD5']) {
+            $command[$prefix . 'SSECustomerKeyMD5'] = \base64_encode($md5);
+        } else {
+            $command[$prefix . 'SSECustomerKeyMD5'] = \base64_encode(\md5($key, \true));
+        }
+    }
+}