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/AssetsSettings.svelte

@@ -0,0 +1,67 @@
+<script>
+	import {strings, urls} from "../js/stores";
+	import {assetsSettings, assetsDefinedSettings} from "./stores";
+	import AssetsSettingsHeaderRow from "./AssetsSettingsHeaderRow.svelte";
+	import Panel from "../components/Panel.svelte";
+	import SettingsPanelOption from "../components/SettingsPanelOption.svelte";
+	import SettingsValidationStatusRow
+		from "../components/SettingsValidationStatusRow.svelte";
+
+	/**
+	 * Potentially returns a reason that the provided domain name is invalid.
+	 *
+	 * @param {string} domain
+	 *
+	 * @return {string}
+	 */
+	function validator( domain ) {
+		const domainPattern = /[^a-z0-9.-]/;
+
+		let message = "";
+
+		if ( domain.trim().length === 0 ) {
+			message = $strings.domain_blank;
+		} else if ( true === domainPattern.test( domain ) ) {
+			message = $strings.domain_invalid_content;
+		} else if ( domain.length < 3 ) {
+			message = $strings.domain_too_short;
+		} else if ( domain === $urls.home_domain ) {
+			message = $strings.assets_domain_same_as_site;
+		}
+
+		return message;
+	}
+</script>
+
+<Panel name="settings" class="assets-panel" heading={$strings.assets_title} helpKey="assets-pull">
+	<AssetsSettingsHeaderRow/>
+	<SettingsValidationStatusRow section="assets"/>
+	<SettingsPanelOption
+		heading={$strings.assets_rewrite_urls}
+		description={$strings.assets_rewrite_urls_desc}
+		placeholder="assets.example.com"
+		toggleName="rewrite-urls"
+		bind:toggle={$assetsSettings["rewrite-urls"]}
+		textName="domain"
+		bind:text={$assetsSettings["domain"]}
+		definedSettings={assetsDefinedSettings}
+		{validator}
+	>
+	</SettingsPanelOption>
+
+	<SettingsPanelOption
+		heading={$strings.assets_force_https}
+		description={$strings.assets_force_https_desc}
+		toggleName="force-https"
+		bind:toggle={$assetsSettings["force-https"]}
+		definedSettings={assetsDefinedSettings}
+	/>
+</Panel>
+
+<!--
+<div class="btn-row">
+	<div class="notice">
+		<img class="icon notice-icon assets-wizard" src="{$urls.assets + 'img/icon/assets-wizard.svg'}" alt="Launch the Assets Setup Wizard"/><a href={$urls.settings} class="link">Launch the Assets Setup Wizard</a>
+	</div>
+</div>
+-->