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/provider/storage/s3compatible-round.svg

@@ -0,0 +1,7 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 80 80" fill="none">
+  <circle cx="40" cy="40" r="38" fill="#3B5BDB"/>
+  <rect x="16" y="26" width="48" height="30" rx="3" fill="white" opacity="0.15"/>
+  <rect x="16" y="26" width="48" height="30" rx="3" stroke="white" stroke-width="2"/>
+  <ellipse cx="40" cy="26" rx="24" ry="6" fill="white"/>
+  <text x="40" y="49" font-family="monospace,sans-serif" font-size="12" font-weight="700" fill="white" text-anchor="middle">S3</text>
+</svg>