Summary

Replace the inline file upload for the SEO social preview image in page
settings with the existing file manager, so users pick from their asset
library instead of uploading on the fly.

Changes

• Replace hidden file input and upload flow with openFileManager from the editor store
• Remove pendingImageFile, imagePreviewUrl, uploadedAssetCache state and all related cleanup logic
• Remove asset upload/deletion during save — selected assets are shared and managed through the file manager
• Rename "Upload image" button to "Choose image"
• Remove unused uploadFileApi, deleteAssetApi, useAssetsStore imports

Test plan

• Open page settings > SEO tab > click "Choose image" — file manager opens filtered to images
• Select an image — preview displays in the social preview area
• Click "Replace" — filr reopens with current asset highlighted
• Click "Remove" — image clears
• Save and reopen — selected image persists
• Existing pages with previously uploaded SEO images still display correctly
• Dynamic pages: "Select field" option still works alongside "Choose image"