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/Panel.svelte

@@ -0,0 +1,139 @@
+<script>
+	import {createEventDispatcher, getContext, hasContext} from "svelte";
+	import {writable} from "svelte/store";
+	import {fade} from "svelte/transition";
+	import {link} from "svelte-spa-router";
+	import {defined_settings, strings} from "../js/stores";
+	import PanelContainer from "./PanelContainer.svelte";
+	import PanelRow from "./PanelRow.svelte";
+	import DefinedInWPConfig from "./DefinedInWPConfig.svelte";
+	import ToggleSwitch from "./ToggleSwitch.svelte";
+	import HelpButton from "./HelpButton.svelte";
+	import Button from "./Button.svelte";
+
+	const classes = $$props.class ? $$props.class : "";
+	const dispatch = createEventDispatcher();
+
+	export let ref = {};
+	export let name = "";
+	export let heading = "";
+	export let defined = false;
+	export let multi = false;
+	export let flyout = false;
+	export let toggleName = "";
+	export let toggle = false;
+	export let refresh = false;
+	export let refreshText = $strings.refresh_title;
+	export let refreshDesc = refreshText;
+	export let refreshing = false;
+	export let helpKey = "";
+	export let helpURL = "";
+	export let helpDesc = $strings.help_desc;
+
+	// We can display storage provider info on the right-hand side of the panel's header.
+	// In the future, if anything else needs to be displayed in the same position we
+	// should create a named slot or assignable component. CSS changes would be required.
+	export let storageProvider = null;
+
+	// Parent page may want to be locked.
+	let settingsLocked = writable( false );
+
+	if ( hasContext( "settingsLocked" ) ) {
+		settingsLocked = getContext( "settingsLocked" );
+	}
+
+	$: locked = $settingsLocked;
+	$: toggleDisabled = $defined_settings.includes( toggleName ) || locked;
+
+	/**
+	 * If appropriate, clicking the header toggles to toggle switch.
+	 */
+	function headingClickHandler() {
+		if ( toggleName && !toggleDisabled ) {
+			toggle = !toggle;
+		}
+	}
+
+	/**
+	 * Catch escape key and emit a custom cancel event.
+	 *
+	 * @param {KeyboardEvent} event
+	 */
+	function handleKeyup( event ) {
+		if ( event.key === "Escape" ) {
+			event.preventDefault();
+			dispatch( "cancel" );
+		}
+	}
+</script>
+
+<!-- TODO: Fix a11y. -->
+<!-- svelte-ignore a11y-no-static-element-interactions -->
+<div
+	class="panel {name}"
+	class:multi
+	class:flyout
+	class:locked
+	transition:fade={{duration: flyout ? 200 : 0}}
+	bind:this={ref}
+	on:focusout
+	on:mouseenter
+	on:mouseleave
+	on:mousedown
+	on:click
+	on:keyup={handleKeyup}
+>
+	{#if !multi && heading}
+		<div class="heading">
+			<h2>{heading}</h2>
+			{#if helpURL}
+				<HelpButton url={helpURL} desc={helpDesc}/>
+			{:else if helpKey}
+				<HelpButton key={helpKey} desc={helpDesc}/>
+			{/if}
+			<DefinedInWPConfig {defined}/>
+		</div>
+	{/if}
+	<PanelContainer class={classes}>
+		{#if multi && heading}
+			<PanelRow header>
+				{#if toggleName}
+					<ToggleSwitch name={toggleName} bind:checked={toggle} disabled={toggleDisabled}>
+						{heading}
+					</ToggleSwitch>
+					<!-- TODO: Fix a11y. -->
+					<!-- svelte-ignore a11y-no-noninteractive-element-interactions -->
+					<!-- svelte-ignore a11y-click-events-have-key-events -->
+					<h3 on:click={headingClickHandler} class="toggler" class:toggleDisabled>{heading}</h3>
+				{:else}
+					<h3>{heading}</h3>
+				{/if}
+				<DefinedInWPConfig {defined}/>
+				{#if refresh}
+					<Button refresh {refreshing} title={refreshDesc} on:click={() => dispatch("refresh")}>{@html refreshText}</Button>
+				{/if}
+				{#if storageProvider}
+					<div class="provider">
+						<a href="/storage/provider" use:link class="link">
+							<img src={storageProvider.link_icon} alt={storageProvider.icon_desc}>
+							{storageProvider.provider_service_name}
+						</a>
+					</div>
+				{/if}
+				{#if helpURL}
+					<HelpButton url={helpURL} desc={helpDesc}/>
+				{:else if helpKey}
+					<HelpButton key={helpKey} desc={helpDesc}/>
+				{/if}
+			</PanelRow>
+		{/if}
+
+		<slot/>
+	</PanelContainer>
+</div>
+
+<style>
+	.toggler:not(.toggleDisabled) {
+		cursor: pointer;
+	}
+</style>