Fix block styles missing in pattern creator editor canvas

[bor0]

Summary

• Enqueues block frontend style_handles via enqueue_block_assets so they are captured by _wp_get_iframed_editor_assets, which only picks up editor_style_handles and misses frontend styles needed by blocks like Cover for layout/background rendering.
• Moves EditorStyles from the head prop of Iframe to inside the component as a child, so styles are injected within the canvas iframe.

Fixes #737

Test plan

• Open the pattern creator and insert a Cover block — verify background/layout styles render correctly in the editor canvas.
• Confirm no regressions in other blocks' styles inside the canvas.
• Verify the editor still loads without JS errors.

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