fix(posts-inserter): show placeholder when block has zero posts

[dkoo]

All Submissions:

• Have you followed the Newspack Contributing guidelines?
• Does your code follow the WordPress coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Shows a placeholder when the Posts Inserter block has zero posts to display. This can easily happen by selecting "Display specific posts" and not selecting any posts, or by choosing a combination of post types and category filters that match zero posts.

Closes NPPM-2870.

How to test the changes in this Pull Request:

While on release:

1. In a Newsletter post, add a Post Inserter block
2. Select "Display specific posts"
3. Observe that the placeholder collapses to a solid line
4. Click away from the block
5. Observe difficulty in interacting with the block through the post editor UI

Now check out this branch

6. Refresh the newsletter editor and repeat
7. Confirm that the block now shows a placeholder with message in the post editor:

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully run tests with your changes locally?

[Copilot]

Pull request overview

This PR improves the Posts Inserter block editor experience by showing a clear placeholder when the current post selection or filters produce zero posts, preventing the preview area from collapsing.

Changes:

• Adds localized placeholder text and icon imports.
• Renders a spinner, block preview, or “No posts found” placeholder based on readiness and block count.