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/img/icon/bucket-round.svg

@@ -0,0 +1,6 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="42" height="42" viewBox="0 0 42 42">
+  <g fill="none" fill-rule="evenodd">
+    <circle cx="21" cy="21" r="21" fill="#E4EAF1"/>
+    <path fill="#87A2C2" fill-rule="nonzero" d="M27.5,18 L26,18 C25.8603036,15.5110271 24.5656512,11 21.0000303,11 C17.4344094,11 16.139757,15.5110271 16,18 L15,18 C15,18 14.5,18 14.5,18 C14.5,18 14,18 14,18.5 C14,19 14.5,19 14.5,19 C14.5,19 15,19 15,19 L15.8824161,26 L26.1175839,26 L27,19 L27.5,19 C27.7976351,19 28,18.7960737 28,18.5 C28,18.2039263 27.7976351,18 27.5,18 Z M17,18 C17.1205053,16.3757768 17.7470684,12 21,12 C24.2529316,12 24.8794947,16.3757768 25,18 L17,18 Z M16.1665769,28.5497889 C16.1938559,28.8051787 16.4247894,29 16.700845,29 L25.2990945,29 C25.5751502,29 25.8060231,28.8051787 25.8333626,28.5497889 L26,27 L16,27 L16.1665769,28.5497889 Z"/>
+  </g>
+</svg>