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

ui/pro/AssetsPage.svelte

@@ -0,0 +1,33 @@
+<script>
+	import {
+		assetsSettings,
+		assetsSettingsChanged,
+		assetsSettingsLocked,
+		currentAssetsSettings,
+		enableAssets
+	} from "./stores";
+	import Page from "../components/Page.svelte";
+	import Notifications from "../components/Notifications.svelte";
+	import AssetsSettings from "./AssetsSettings.svelte";
+	import AssetsUpgrade from "./AssetsUpgrade.svelte";
+	import Footer from "../components/Footer.svelte";
+	import {setContext} from "svelte";
+
+	export let name = "assets";
+
+	// Let all child components know if settings are currently locked.
+	setContext( "settingsLocked", assetsSettingsLocked );
+</script>
+
+<Page {name} on:routeEvent initialSettings={currentAssetsSettings}>
+	<Notifications tab={name}/>
+	<div class="assets-page wrapper">
+		{#if $enableAssets}
+			<AssetsSettings/>
+		{:else}
+			<AssetsUpgrade/>
+		{/if}
+	</div>
+</Page>
+
+<Footer settingsStore={assetsSettings} settingsChangedStore={assetsSettingsChanged} on:routeEvent/>