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/Crypto/Polyfill/Key.php

@@ -0,0 +1,71 @@
+<?php
+
+namespace DeliciousBrains\WP_Offload_Media\Aws3\Aws\Crypto\Polyfill;
+
+/**
+ * Class Key
+ *
+ * Wraps a string to keep it hidden from stack traces.
+ */
+class Key
+{
+    /**
+     * @var string $internalString
+     */
+    private $internalString;
+    /**
+     * Hide contents of 
+     *
+     * @return array
+     */
+    public function __debugInfo()
+    {
+        return [];
+    }
+    /**
+     * Key constructor.
+     * @param string $str
+     */
+    public function __construct($str)
+    {
+        $this->internalString = $str;
+    }
+    /**
+     * Defense in depth:
+     *
+     * PHP 7.2 includes the Sodium cryptography library, which (among other things)
+     * exposes a function called sodium_memzero() that we can use to zero-fill strings
+     * to minimize the risk of sensitive cryptographic materials persisting in memory.
+     *
+     * If this function is not available, we XOR the string in-place with itself as a
+     * best-effort attempt.
+     */
+    public function __destruct()
+    {
+        if (\extension_loaded('sodium') && \function_exists('sodium_memzero')) {
+            try {
+                \sodium_memzero($this->internalString);
+            } catch (\SodiumException $ex) {
+                // This is a best effort, but does not provide the same guarantees as sodium_memzero():
+                $this->internalString ^= $this->internalString;
+            }
+        }
+    }
+    /**
+     * @return string
+     */
+    public function get()
+    {
+        return $this->internalString;
+    }
+    /**
+     * @return int
+     */
+    public function length()
+    {
+        if (\is_callable('\\mb_strlen')) {
+            return (int) \mb_strlen($this->internalString, '8bit');
+        }
+        return (int) \strlen($this->internalString);
+    }
+}