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/Footer.svelte

@@ -0,0 +1,75 @@
+<script>
+	import {createEventDispatcher, onDestroy} from "svelte";
+	import {slide} from "svelte/transition";
+	import {
+		revalidatingSettings,
+		settings_changed,
+		settings,
+		strings,
+		state,
+		validationErrors
+	} from "../js/stores";
+	import {
+		scrollNotificationsIntoView
+	} from "../js/scrollNotificationsIntoView";
+	import Button from "./Button.svelte";
+
+	const dispatch = createEventDispatcher();
+
+	export let settingsStore = settings;
+	export let settingsChangedStore = settings_changed;
+
+	let saving = false;
+
+	$: disabled = saving || $validationErrors.size > 0;
+
+	// On init, start with no validation errors.
+	validationErrors.set( new Map() );
+
+	/**
+	 * Handles a Cancel button click.
+	 */
+	function handleCancel() {
+		settingsStore.reset();
+	}
+
+	/**
+	 * Handles a Save button click.
+	 *
+	 * @return {Promise<void>}
+	 */
+	async function handleSave() {
+		saving = true;
+		state.pausePeriodicFetch();
+		const result = await settingsStore.save();
+		$revalidatingSettings = true;
+		const statePromise = state.resumePeriodicFetch();
+
+		// The save happened, whether anything changed or not.
+		if ( result.hasOwnProperty( "saved" ) && result.hasOwnProperty( "changed_settings" ) ) {
+			dispatch( "routeEvent", { event: "settings.save", data: result } );
+		}
+
+		// After save make sure notifications are eyeballed.
+		scrollNotificationsIntoView();
+		saving = false;
+
+		// Just make sure periodic state fetch promise is done with,
+		// even though we don't really care about it.
+		await statePromise;
+		$revalidatingSettings = false;
+	}
+
+	// On navigation away from a component showing the footer,
+	// make sure settings are reset.
+	onDestroy( () => handleCancel() );
+</script>
+
+{#if $settingsChangedStore}
+	<div class="fixed-cta-block" transition:slide>
+		<div class="buttons">
+			<Button outline on:click={handleCancel}>{$strings.cancel_button}</Button>
+			<Button primary on:click={handleSave} {disabled}>{$strings.save_changes}</Button>
+		</div>
+	</div>
+{/if}