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/components/UrlPreview.svelte

@@ -0,0 +1,59 @@
+<script>
+	import {scale} from "svelte/transition";
+	import {api, settings, settings_changed, strings, urls} from "../js/stores";
+	import Panel from "./Panel.svelte";
+	import PanelRow from "./PanelRow.svelte";
+
+	let parts = $urls.url_parts;
+
+	/**
+	 * When settings have changed, show their preview URL, otherwise show saved settings version.
+	 *
+	 * Note: This function **assigns** to the `example` and `parts` variables to defeat the reactive demons!
+	 *
+	 * @param {Object} urls
+	 * @param {boolean} settingsChanged
+	 * @param {Object} settings
+	 *
+	 * @returns boolean
+	 */
+	async function temporaryUrl( urls, settingsChanged, settings ) {
+		if ( settingsChanged ) {
+			const response = await api.post( "url-preview", {
+				"settings": settings
+			} );
+
+			// Use temporary URLs if available.
+			if ( response.hasOwnProperty( "url_parts" ) ) {
+				parts = response.url_parts;
+
+				return true;
+			}
+		}
+
+		// Reset back to saved URLs.
+		parts = urls.url_parts;
+
+		return false;
+	}
+
+	$: isTemporaryUrl = temporaryUrl( $urls, $settings_changed, $settings );
+</script>
+
+{#if parts.length > 0}
+	<Panel name="url-preview" heading={$strings.url_preview_title}>
+		<PanelRow class="desc">
+			<p>{$strings.url_preview_desc}</p>
+		</PanelRow>
+		<PanelRow class="body flex-row">
+			<dl>
+				{#each parts as part (part.title)}
+					<div data-key={part.key} transition:scale>
+						<dt>{part.title}</dt>
+						<dd>{part.example}</dd>
+					</div>
+				{/each}
+			</dl>
+		</PanelRow>
+	</Panel>
+{/if}