fix(media): use correct media type labels for block protection toolbar

[faisalahammad]

Summary

The media protection toolbar button and modal always said "Protect Image" even when the selected block was a file, video, or audio block. This change makes the label match the actual media type.

Fixes #3165

Changes

src/js/admin-media-protection-block-protect.js

Before:

<ToolbarButton
    icon="lock"
    label="Protect Image"
    onClick={ () => setModalOpen( true ) }
/>

After:

const mediaTypeLabel = getMediaTypeLabel( props.name );

<ToolbarButton
    icon="lock"
    label={ LLMS.l10n.replace( 'Protect %s', { '%s': mediaTypeLabel } ) }
    onClick={ () => setModalOpen( true ) }
/>

Why: core/image, core/video, core/audio, and core/file are all supported, but the labels were hardcoded for images. A new getMediaTypeLabel() helper returns the correct noun for each block type.

includes/admin/class.llms.admin.assets.php

Before:
Core did not provide the translatable strings used by the new dynamic labels, so the toolbar would fall back to the literal English strings.

After:
Core now adds the needed strings via the existing lifterlms_js_l10n_admin filter:

public function add_media_protection_js_l10n( $strings ) {
    return array_merge(
        $strings,
        array(
            'Protect %s' => __( 'Protect %s', 'lifterlms' ),
            'Select a Course or Membership to protect this %s:' => __( 'Select a Course or Membership to protect this %s:', 'lifterlms' ),
            'image' => __( 'image', 'lifterlms' ),
            'audio' => __( 'audio', 'lifterlms' ),
            'video' => __( 'video', 'lifterlms' ),
            'file' => __( 'file', 'lifterlms' ),
        )
    );
}

Why: This keeps the change inside the existing LLMS.l10n system instead of introducing a new wp-i18n dependency for one script.

Testing

Test 1: File block label

1. Create a page and add a File block with an uploaded file.
2. Click the lock icon in the toolbar.

Result: toolbar tooltip and modal button say "Protect File", and the select label says "Select a Course or Membership to protect this file:".

Test 2: Video and Audio blocks

1. Add a Video block and an Audio block.
2. Click the lock icon for each.

Result: labels say "Protect Video" and "Protect Audio" respectively.

Test 3: Image block remains unchanged

1. Add an Image block.
2. Click the lock icon.

Result: labels still say "Protect Image".