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/SettingsPanelOption.svelte

@@ -0,0 +1,152 @@
+<script>
+	import {getContext, hasContext} from "svelte";
+	import {writable} from "svelte/store";
+	import {slide} from "svelte/transition";
+	import {defined_settings, validationErrors} from "../js/stores";
+	import PanelRow from "./PanelRow.svelte";
+	import ToggleSwitch from "./ToggleSwitch.svelte";
+	import DefinedInWPConfig from "./DefinedInWPConfig.svelte";
+	import SettingNotifications from "./SettingNotifications.svelte";
+
+	export let heading = "";
+	export let description = "";
+	export let placeholder = "";
+	export let nested = false;
+	export let first = false; // of nested items
+
+	// Toggle and Text may both be used at same time.
+	export let toggleName = "";
+	export let toggle = false;
+	export let textName = "";
+	export let text = "";
+	export let alwaysShowText = false;
+
+	export let definedSettings = defined_settings;
+
+	/**
+	 * Optional validator function.
+	 *
+	 * @param {string} textValue
+	 *
+	 * @return {string} Empty if no error
+	 */
+	export let validator = ( textValue ) => "";
+
+	// Parent page may want to be locked.
+	let settingsLocked = writable( false );
+
+	let textDirty = false;
+
+	if ( hasContext( "settingsLocked" ) ) {
+		settingsLocked = getContext( "settingsLocked" );
+	}
+
+	$: locked = $settingsLocked;
+	$: toggleDisabled = $definedSettings.includes( toggleName ) || locked;
+	$: textDisabled = $definedSettings.includes( textName ) || locked;
+
+	$: input = ((toggleName && toggle) || !toggleName || alwaysShowText) && textName;
+	$: headingName = input ? textName + "-heading" : toggleName;
+
+	/**
+	 * Validate the text if validator function supplied.
+	 *
+	 * @param {string} text
+	 * @param {bool} toggle
+	 *
+	 * @return {string}
+	 */
+	function validateText( text, toggle ) {
+		let message = "";
+
+		if ( validator !== undefined && toggle && !textDisabled ) {
+			message = validator( text );
+		}
+
+		validationErrors.update( _validationErrors => {
+			if ( _validationErrors.has( textName ) && message === "" ) {
+				_validationErrors.delete( textName );
+			} else if ( message !== "" ) {
+				_validationErrors.set( textName, message );
+			}
+
+			return _validationErrors;
+		} );
+
+		return message;
+	}
+
+	function onTextInput() {
+		textDirty = true;
+	}
+
+	$: validationError = validateText( text, toggle );
+
+	/**
+	 * If appropriate, clicking the header toggles to toggle switch.
+	 */
+	function headingClickHandler() {
+		if ( toggleName && !toggleDisabled ) {
+			toggle = !toggle;
+		}
+	}
+</script>
+
+<div class="setting" class:nested class:first>
+	<PanelRow class="option">
+		{#if toggleName}
+			<ToggleSwitch name={toggleName} bind:checked={toggle} disabled={toggleDisabled}>
+				{heading}
+			</ToggleSwitch>
+			<!-- TODO: Fix a11y. -->
+			<!-- svelte-ignore a11y-no-noninteractive-element-interactions -->
+			<!-- svelte-ignore a11y-click-events-have-key-events -->
+			<h4 id={headingName} on:click={headingClickHandler} class="toggler" class:toggleDisabled>{heading}</h4>
+		{:else}
+			<h4 id={headingName}>{heading}</h4>
+		{/if}
+		<DefinedInWPConfig defined={$definedSettings.includes( toggleName ) || (input && $definedSettings.includes( textName ))}/>
+	</PanelRow>
+	<PanelRow class="desc">
+		<p>{@html description}</p>
+	</PanelRow>
+	{#if input}
+		<PanelRow class="input">
+			<input
+				type="text"
+				id={textName}
+				name={textName}
+				bind:value={text}
+				on:input={onTextInput}
+				minlength="1"
+				size="10"
+				{placeholder}
+				disabled={textDisabled}
+				class:disabled={textDisabled}
+				aria-labelledby={headingName}
+			>
+			<label for={textName}>
+				{heading}
+			</label>
+		</PanelRow>
+		{#if validationError && textDirty}
+			<p class="input-error" transition:slide>{validationError}</p>
+		{/if}
+	{/if}
+
+	{#if toggleName}
+		<SettingNotifications settingKey={toggleName}/>
+	{/if}
+
+	{#if textName}
+		<SettingNotifications settingKey={textName}/>
+	{/if}
+
+	<slot/>
+</div>
+
+<style>
+	.toggler:not(.toggleDisabled) {
+		cursor: pointer;
+	}
+</style>