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

assets/css/modal.css

@@ -0,0 +1,73 @@
+/**
+ * Modals
+ */
+#as3cf-overlay {
+  display: none;
+  position: fixed;
+  top: 0;
+  right: 0;
+  bottom: 0;
+  left: 0;
+  background-color: rgba(0, 0, 0, 0.5);
+  overflow: hidden;
+  overflow-y: auto;
+  z-index: 999999;
+}
+
+#as3cf-modal {
+  display: none;
+  position: relative;
+  width: 600px;
+  margin: 100px auto;
+  padding: 30px;
+  background-color: #eee;
+  box-shadow: 0 0 10px rgba(0, 0, 0, 0.5);
+  font-size: 14px;
+  overflow: hidden;
+  z-index: 100000;
+}
+#as3cf-modal .close-as3cf-modal {
+  color: #999;
+  cursor: pointer;
+  font-family: "Times New Roman", serif;
+  font-size: 26px;
+  font-weight: 200;
+  position: absolute;
+  right: 18px;
+  top: 18px;
+}
+#as3cf-modal .close-as3cf-modal:hover {
+  color: #666;
+}
+#as3cf-modal h3 {
+  margin: 0 0 20px;
+  font-weight: normal;
+  line-height: 1;
+}
+#as3cf-modal .error,
+#as3cf-modal .notice,
+#as3cf-modal .updated {
+  margin: 0 0 20px;
+}
+#as3cf-modal .actions {
+  margin: 20px -30px -30px;
+  padding: 20px 30px;
+  border-top: none;
+  background-color: #e3e3e3;
+  overflow: hidden;
+}
+#as3cf-modal .actions .right {
+  margin-left: 15px;
+}
+#as3cf-modal .actions .right:last-of-type {
+  margin-left: 0;
+}
+#as3cf-modal .actions button {
+  min-width: 90px;
+}
+
+body.as3cf-modal-open {
+  overflow: hidden;
+}
+
+/*# sourceMappingURL=modal.css.map */