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/BackNextButtonsRow.svelte

@@ -0,0 +1,57 @@
+<script>
+	import {createEventDispatcher} from "svelte";
+	import {strings} from "../js/stores";
+	import Button from "./Button.svelte";
+
+	const dispatch = createEventDispatcher();
+
+	// Back button is optional but shown by default.
+	export let backText = $strings.back;
+	export let backDisabled = false;
+	export let backTitle = "";
+	export let backVisible = false;
+
+	// Skip button is optional, only shown if explicitly made visible.
+	export let skipText = $strings.skip;
+	export let skipDisabled = false;
+	export let skipTitle = "";
+	export let skipVisible = false;
+
+	// Next button required.
+	export let nextText = $strings.next;
+	export let nextDisabled = false;
+	export let nextTitle = "";
+</script>
+
+<div class="btn-row">
+	{#if backVisible}
+		<Button
+			large
+			on:click="{() => dispatch('back')}"
+			disabled={backDisabled}
+			title={backTitle}
+		>
+			{backText}
+		</Button>
+	{/if}
+	{#if skipVisible}
+		<Button
+			large
+			outline
+			on:click="{() => dispatch('skip')}"
+			disabled={skipDisabled}
+			title={skipTitle}
+		>
+			{skipText}
+		</Button>
+	{/if}
+	<Button
+		large
+		primary
+		on:click="{() => dispatch('next')}"
+		disabled={nextDisabled}
+		title={nextTitle}
+	>
+		{nextText}
+	</Button>
+</div>