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/SignatureInterface.php

@@ -0,0 +1,36 @@
+<?php
+
+namespace DeliciousBrains\WP_Offload_Media\Aws3\Aws\Signature;
+
+use DeliciousBrains\WP_Offload_Media\Aws3\Aws\Credentials\CredentialsInterface;
+use DeliciousBrains\WP_Offload_Media\Aws3\Psr\Http\Message\RequestInterface;
+/**
+ * Interface used to provide interchangeable strategies for signing requests
+ * using the various AWS signature protocols.
+ */
+interface SignatureInterface
+{
+    /**
+     * Signs the specified request with an AWS signing protocol by using the
+     * provided AWS account credentials and adding the required headers to the
+     * request.
+     *
+     * @param RequestInterface     $request     Request to sign
+     * @param CredentialsInterface $credentials Signing credentials
+     *
+     * @return RequestInterface Returns the modified request.
+     */
+    public function signRequest(RequestInterface $request, CredentialsInterface $credentials);
+    /**
+     * Create a pre-signed request.
+     *
+     * @param RequestInterface              $request     Request to sign
+     * @param CredentialsInterface          $credentials Credentials used to sign
+     * @param int|string|\DateTimeInterface $expires The time at which the URL should
+     *     expire. This can be a Unix timestamp, a PHP DateTime object, or a
+     *     string that can be evaluated by strtotime.
+     *
+     * @return RequestInterface
+     */
+    public function presign(RequestInterface $request, CredentialsInterface $credentials, $expires, array $options = []);
+}