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/GuzzleHttp/Psr7/DroppingStream.php

@@ -0,0 +1,40 @@
+<?php
+
+declare (strict_types=1);
+namespace DeliciousBrains\WP_Offload_Media\Aws3\GuzzleHttp\Psr7;
+
+use DeliciousBrains\WP_Offload_Media\Aws3\Psr\Http\Message\StreamInterface;
+/**
+ * Stream decorator that begins dropping data once the size of the underlying
+ * stream becomes too full.
+ */
+final class DroppingStream implements StreamInterface
+{
+    use StreamDecoratorTrait;
+    /** @var int */
+    private $maxLength;
+    /** @var StreamInterface */
+    private $stream;
+    /**
+     * @param StreamInterface $stream    Underlying stream to decorate.
+     * @param int             $maxLength Maximum size before dropping data.
+     */
+    public function __construct(StreamInterface $stream, int $maxLength)
+    {
+        $this->stream = $stream;
+        $this->maxLength = $maxLength;
+    }
+    public function write($string) : int
+    {
+        $diff = $this->maxLength - $this->stream->getSize();
+        // Begin returning 0 when the underlying stream is too large.
+        if ($diff <= 0) {
+            return 0;
+        }
+        // Write the stream or a subset of the stream if needed.
+        if (\strlen($string) < $diff) {
+            return $this->stream->write($string);
+        }
+        return $this->stream->write(\substr($string, 0, $diff));
+    }
+}